In quantum mechanics, the Schrödinger equation is a partial differential equation that describes how the quantum state of some physical system changes with time. It was formulated in late 1925, and published in 1926, by the Austrian physicist Erwin Schrödinger.
In classical mechanics, the equation of motion is Newton's second law, and equivalent formulations are the Euler–Lagrange equations and Hamilton's equations. All of these formulations are used to solve for the motion of a mechanical system and mathematically predict what the system will do at any time beyond the initial settings and configuration of the system.
In quantum mechanics, the analogue of Newton's law is Schrödinger's equation for a quantum system (usually atoms, molecules, and subatomic particles whether free, bound, or localized). It is not a simple algebraic equation, but (in general) a linear partial differential equation. The differential equation describes the wave function of the system, also called the quantum state or state vector.
The concept of a state vector, and an equation governing its time evolution, namely the Schrödinger equation, are fundamental postulates of quantum mechanics. These ideas cannot be derived from any other principle.
In the standard interpretation of quantum mechanics, the wave function is the most complete description that can be given to a physical system. Solutions to Schrödinger's equation describe not only molecular, atomic, and subatomic systems, but also macroscopic systems, possibly even the whole universe.^{[1]}
Like Newton's Second law, the Schrödinger equation can be mathematically transformed into other formulations such as Werner Heisenberg's matrix mechanics, and Richard Feynman's path integral formulation. Also like Newton's Second law, the Schrödinger equation describes time in a way that is inconvenient for relativistic theories, a problem that is not as severe in matrix mechanics and completely absent in the path integral formulation. The equation is derived by partially differentiating the standard wave equation and substituting the relation between the momentum of the particle and the wavelength of the wave associated with the particle in De Broglie's hypothesis.
Equation
Timedependent equation
The form of the Schrödinger equation depends on the physical situation (see below for special cases). The most general form is the timedependent Schrödinger equation, which gives a description of a system evolving with time:^{[2]}
where i is the imaginary unit, the symbol "∂/∂t" indicates a partial derivative with respect to time t, ħ is Planck's constant divided by 2π, Ψ is the wave function of the quantum system, and $\backslash hat\{H\}$ is the Hamiltonian operator (which characterizes the total energy of any given wave function and takes different forms depending on the situation).
The most famous example is the nonrelativistic Schrödinger equation for a single particle moving in an electric field (but not a magnetic field; see the Pauli equation):
where m is the particle's mass, V is its potential energy, ∇^{2} is the Laplacian, and Ψ is the wave function (more precisely, in this context, it is called the "positionspace wave function"). In plain language, it means "total energy equals kinetic energy plus potential energy", but the terms take unfamiliar forms for reasons explained below.
Given the particular differential operators involved, this is a linear partial differential equation. It is also a diffusion equation, but unlike the heat equation, this one is also a wave equation given the imaginary unit present in the transient term.
The term "Schrödinger equation" can refer to both the general equation (first box above), or the specific nonrelativistic version (second box above and variations thereof). The general equation is indeed quite general, used throughout quantum mechanics, for everything from the Dirac equation to quantum field theory, by plugging in various complicated expressions for the Hamiltonian. The specific nonrelativistic version is a simplified approximation to reality, which is quite accurate in many situations, but very inaccurate in others (see relativistic quantum mechanics and relativistic quantum field theory).
To apply the Schrödinger equation, the Hamiltonian operator is set up for the system, accounting for the kinetic and potential energy of the particles constituting the system, then inserted into the Schrödinger equation. The resulting partial differential equation is solved for the wave function, which contains information about the system.
Timeindependent equation
The timedependent Schrödinger equation predicts that wave functions can form standing waves, called stationary states (also called "orbitals", as in atomic orbitals or molecular orbitals). These states are important in their own right, and moreover if the stationary states are classified and understood, then it becomes easier to solve the timedependent Schrödinger equation for any state. The timeindependent Schrödinger equation is the equation describing stationary states. (It is only used when the Hamiltonian itself is not dependent on time.)
In words, the equation states:
 When the Hamiltonian operator acts on a certain wave function Ψ, and the result is proportional to the same wave function Ψ, then Ψ is a stationary state, and the proportionality constant, E, is the energy of the state Ψ.
The timeindependent Schrödinger equation is discussed further below. In linear algebra terminology, this equation is an eigenvalue equation.
As before, the most famous manifestation is the nonrelativistic Schrödinger equation for a single particle moving in an electric field (but not a magnetic field):
with definitions as above.
Implications
The Schrödinger equation, and its solutions, introduced a breakthrough in thinking about physics. Schrödinger's equation was the first of its type, and solutions led to very unusual and unexpected consequences for the time.
Total, kinetic, and potential energy
The overall form of the equation is not unusual or unexpected as it uses the principle of the conservation of energy. The terms of the nonrelativistic Schrödinger equation can be interpreted as total energy of the system, equal to the system kinetic energy plus the system potential energy. In this respect, it is just the same as in classical physics.
Quantization
The Schrödinger equation predicts that if certain properties of a system are measured, the result may be quantized, meaning that only specific discrete values can occur. One example is energy quantization: the energy of an electron in an atom is always one of the quantized energy levels, a fact discovered via atomic spectroscopy. (Energy quantization is discussed below.) Another example is quantization of angular momentum. This was an assumption in the earlier Bohr model of the atom, but it is a prediction of the Schrödinger equation.
Another result of the Schrödinger equation is that not every measurement gives a quantized result in quantum mechanics. For example, position, momentum, time, and (in some situations) energy can have any value across a continuous range.
Measurement and uncertainty
In classical mechanics, a particle has, at every moment, an exact position and an exact momentum. These values change deterministically as the particle moves according to Newton's laws. In quantum mechanics, particles do not have exactly determined properties, and when they are measured, the result is randomly drawn from a probability distribution. The Schrödinger equation predicts what the probability distributions are, but fundamentally cannot predict the exact result of each measurement.
The Heisenberg uncertainty principle is the statement of the inherent measurement uncertainty in quantum mechanics. It states that the more precisely a particle's position is known, the less precisely its momentum is known, and vice versa.
The Schrödinger equation describes the (deterministic) evolution of the wave function of a particle. However, even if the wave function is known exactly, the result of a specific measurement on the wave function is uncertain.
Quantum tunneling
In classical physics, when a ball is rolled slowly up a large hill, it will come to a stop and roll back, because it doesn't have enough energy to get over the top of the hill to the other side. However, the Schrödinger equation predicts that there is a small probability that the ball will get to the other side of the hill, even if it has too little energy to reach the top. This is called quantum tunneling. It is related to the uncertainty principle: Although the ball seems to be on one side of the hill, its position is uncertain so there is a chance of finding it on the other side.
The solution to Schrödinger's equation for a
1d step potential system (dotted line), shown in slices of constant time, the
opacity corresponds to the
probability density 
Ψ
^{2} of the particle (blue circles) at the location shown. The probability of the particle passing through the barrier (
transmission) is greater than it
reflecting from the barrier, since the total energy
E of the particle exceeds the potential energy
V.
^{[3]}
Quantum tunneling through a barrier. A particle coming from the left does not have enough energy to climb the barrier. However, it can sometimes "tunnel" to the other side.
The fuzziness of a particle's position illustrated, which is not definite in quantum mechanics.
Particles as waves
The nonrelativistic Schrödinger equation is a type of partial differential equation called a wave equation. Therefore it is often said particles can exhibit behavior usually attributed to waves. In most modern interpretations this description is reversed – the quantum state, i.e. wave, is the only genuine physical reality, and under the appropriate conditions it can show features of particlelike behavior.
Twoslit diffraction is a famous example of the strange behaviors that waves regularly display, that are not intuitively associated with particles. The overlapping waves from the two slits cancel each other out in some locations, and reinforce each other in other locations, causing a complex pattern to emerge. Intuitively, one would not expect this pattern from firing a single particle at the slits, because the particle should pass through one slit or the other, not a complex overlap of both.
However, since the Schrödinger equation is a wave equation, a single particle fired through a doubleslit does show this same pattern (figure on left). Note: The experiment must be repeated many times for the complex pattern to emerge. The appearance of the pattern proves that each electron passes through both slits simultaneously.^{[4]}^{[5]}^{[6]} Although this is counterintuitive, the prediction is correct; in particular, electron diffraction and neutron diffraction are well understood and widely used in science and engineering.
Related to diffraction, particles also display superposition and interference.
The superposition property allows the particle to be in a quantum superposition of two or more states with different classical properties at the same time. For example, a particle can have several different energies at the same time, and can be in several different locations at the same time. In the above example, a particle can pass through two slits at the same time. This superposition is still a single quantum state, as shown by the interference effects, even though that conflicts with classical intuition.
Propagation of
de Broglie waves in 1d – real part of the
complex amplitude is blue, imaginary part is green. The probability (shown as the color
opacity) of finding the particle at a given point
x is spread out like a waveform; there is no definite position of the particle. As the amplitude increases above zero the
curvature reverses sign, so the amplitude begins decrease again, and vice versa – the result is an alternating amplitude: a wave.
Interpretation of the wave function
The Schrödinger equation provides a way to calculate the possible wave functions of a system and how they dynamically change in time. However, the Schrödinger equation does not directly say what, exactly, the wave function is. Interpretations of quantum mechanics address questions such as what the relation is between the wave function, the underlying reality, and the results of experimental measurements.
An important aspect is the relationship between the Schrödinger equation and wavefunction collapse. In the oldest Copenhagen interpretation, particles follow the Schrödinger equation except during wavefunction collapse, during which they behave entirely differently. The advent of quantum decoherence theory allowed alternative approaches (such as the Everett manyworlds interpretation and consistent histories), wherein the Schrödinger equation is always satisfied, and wavefunction collapse should be explained as a consequence of the Schrödinger equation.
Historical background and development
Following Max Planck's quantization of light (see black body radiation), Albert Einstein interpreted Planck's quanta to be photons, particles of light, and proposed that the energy of a photon is proportional to its frequency, one of the first signs of wave–particle duality. Since energy and momentum are related in the same way as frequency and wavenumber in special relativity, it followed that the momentum p of a photon is inversely proportional to its wavelength λ, or proportional to its wavenumber k.
 $p\; =\; \backslash frac\{h\}\{\backslash lambda\}\; =\; \backslash hbar\; k$
where h is Planck's constant. Louis de Broglie hypothesized that this is true for all particles, even particles such as electrons. He showed that, assuming that the matter waves propagate along with their particle counterparts, electrons form standing waves, meaning that only certain discrete rotational frequencies about the nucleus of an atom are allowed.^{[7]}
These quantized orbits correspond to discrete energy levels, and de Broglie reproduced the Bohr model formula for the energy levels. The Bohr model was based on the assumed quantization of angular momentum L according to:
 $L\; =\; n\{h\; \backslash over\; 2\backslash pi\}\; =\; n\backslash hbar.$
According to de Broglie the electron is described by a wave and a whole number of wavelengths must fit along the circumference of the electron's orbit:
 $n\; \backslash lambda\; =\; 2\; \backslash pi\; r.\backslash ,$
This approach essentially confined the electron wave in one dimension, along a circular orbit of radius r.
In 1921, prior to de Broglie, Arthur C. Lunn at the University of Chicago had used the same argument based on the completion of the relativistic energy–momentum 4vector to derive what we now call the de Broglie relation^{[8]} Unlike de Broglie, Lunn went on to formulate the differential equation now known as the Schrödinger equation, and solve for its energy eigenvalues for the hydrogen atom. Unfortunately the paper was rejected by the Physical Review, as recounted by Kamen.^{[9]}
Following up on de Broglie's ideas, physicist Peter Debye made an offhand comment that if particles behaved as waves, they should satisfy some sort of wave equation. Inspired by Debye's remark, Schrödinger decided to find a proper 3dimensional wave equation for the electron. He was guided by William R. Hamilton's analogy between mechanics and optics, encoded in the observation that the zerowavelength limit of optics resembles a mechanical system — the trajectories of light rays become sharp tracks that obey Fermat's principle, an analog of the principle of least action.^{[10]} A modern version of his reasoning is reproduced below. The equation he found is:^{[11]}
 $i\backslash hbar\; \backslash frac\{\backslash partial\}\{\backslash partial\; t\}\backslash Psi(\backslash mathbf\{r\},\backslash ,t)=\backslash frac\{\backslash hbar^2\}\{2m\}\backslash nabla^2\backslash Psi(\backslash mathbf\{r\},\backslash ,t)\; +\; V(\backslash mathbf\{r\})\backslash Psi(\backslash mathbf\{r\},\backslash ,t).$
However, by that time, Arnold Sommerfeld had refined the Bohr model with relativistic corrections.^{[12]}^{[13]} Schrödinger used the relativistic energy momentum relation to find what is now known as the Klein–Gordon equation in a Coulomb potential (in natural units):
 $\backslash left(E\; +\; \{e^2\backslash over\; r\}\; \backslash right)^2\; \backslash psi(x)\; =\; \; \backslash nabla^2\backslash psi(x)\; +\; m^2\; \backslash psi(x).$
He found the standing waves of this relativistic equation, but the relativistic corrections disagreed with Sommerfeld's formula. Discouraged, he put away his calculations and secluded himself in an isolated mountain cabin with a lover, in December 1925.^{[14]}
While at the cabin, Schrödinger decided that his earlier nonrelativistic calculations were novel enough to publish, and decided to leave off the problem of relativistic corrections for the future. Despite difficulties solving the differential equation for hydrogen (he had later help from his friend the mathematician Hermann Weyl) Schrödinger showed that his nonrelativistic version of the wave equation produced the correct spectral energies of hydrogen in a paper published in 1926.^{[15]} In the equation, Schrödinger computed the hydrogen spectral series by treating a hydrogen atom's electron as a wave Ψ(x, t), moving in a potential well V, created by the proton. This computation accurately reproduced the energy levels of the Bohr model. In a paper, Schrödinger himself explained this equation as follows:
This 1926 paper was enthusiastically endorsed by Einstein, who saw the matterwaves as an intuitive depiction of nature, as opposed to Heisenberg's matrix mechanics, which he considered overly formal.^{[16]}
The Schrödinger equation details the behavior of ψ but says nothing of its nature. Schrödinger tried to interpret it as a charge density in his fourth paper, but he was unsuccessful.^{[17]} In 1926, just a few days after Schrödinger's fourth and final paper was published, Max Born successfully interpreted ψ as the probability amplitude, whose absolute square is equal to probability density.^{[18]} Schrödinger, though, always opposed a statistical or probabilistic approach, with its associated discontinuities—much like Einstein, who believed that quantum mechanics was a statistical approximation to an underlying deterministic theory— and never reconciled with the Copenhagen interpretation.^{[19]}
Louis de Broglie in his later years has proposed a real valued wave function connected to the complex wave function by a proportionality constant and developed the De Broglie–Bohm theory.
The wave equation for particles
The Schrödinger equation was developed principally from the De Broglie hypothesis, a wave equation that would describe particles,^{[20]} and can be constructed in the following way.^{[21]} For a more rigorous mathematical derivation of Schrödinger's equation, see also.^{[22]}
Assumptions
Energy conservation: The total energy E of a particle is the sum of kinetic energy T and potential energy V, this sum is also the frequent expression for the Hamiltonian H in classical mechanics:
 $E\; =\; T\; +\; V\; =H\; \backslash ,\backslash !$
Explicitly, for a particle in one dimension with position x, mass m and momentum p, and potential energy V which generally varies with position and time t:
 $E\; =\; \backslash frac\{p^2\}\{2m\}+V(x,t)=H.$
For three dimensions, the position vector r and momentum vector p must be used:
 $E\; =\; \backslash frac\{\backslash mathbf\{p\}\backslash cdot\backslash mathbf\{p\}\}\{2m\}+V(\backslash mathbf\{r\},t)=H$
This formalism can be extended to any fixed number of particles: the total energy of the system is then the total kinetic energies of the particles, plus the total potential energy, again the Hamiltonian. However, there can be interactions between the particles (an Nbody problem), so the potential energy V can change as the spatial configuration of particles changes, and possibly with time. The potential energy, in general, is not the sum of the separate potential energies for each particle, it is a function of all the spatial positions of the particles. Explicitly:
 $E=\backslash sum\_\{n=1\}^N\; \backslash frac\{\backslash mathbf\{p\}\_n\backslash cdot\backslash mathbf\{p\}\_n\}\{2m\_n\}\; +\; V(\backslash mathbf\{r\}\_1,\backslash mathbf\{r\}\_2\backslash cdots\backslash mathbf\{r\}\_N,t)\; =\; H\; \backslash ,\backslash !$
De Broglie relations: Einstein's light quanta hypothesis (1905) states that the energy E of a photon is proportional to the frequency ν (or angular frequency, ω = 2πν) of the corresponding quantum wavepacket of light:
 $E\; =\; h\backslash nu\; =\; \backslash hbar\; \backslash omega\; \backslash ,\backslash !$
Likewise De Broglie's hypothesis (1924) states that any particle can be associated with a wave, and that the momentum p of the particle is related to the wavelength λ of such a wave, in one dimension, by:
 $p\; =\; \backslash frac\{h\}\{\backslash lambda\}\; =\; \backslash hbar\; k\backslash ;,$
in three dimensions:
 $\backslash mathbf\{p\}\; =\; \backslash frac\{h\}\{\backslash lambda\}\; =\; \backslash hbar\; \backslash mathbf\{k\}\backslash ;.$
where k is the wavevector (wavelength is related to the magnitude of k)
Linearity: The previous assumptions only allow one to derive the equation for plane waves, corresponding to free particles. In general, physical situations are not purely described by plane waves, so for generality the superposition principle is required; any wave can be made by superposition of sinusoidal plane waves. So if the equation is linear, a linear combination of plane waves is also an allowed solution. Hence a necessary and separate requirement is that the Schrödinger equation is a linear differential equation.
Taken together, these attributes mean it should be possible to structure an equation based on the energies of the particles – their possible kinetic and potential energies the system constrains them to have, in terms of some function of the state of the system – the wave function (denoted Ψ). The wave function summarizes the quantum state of the particles in the system, limited by the constraints on the system: the probability the particles are in some spatial configuration at some instant of time. Solving it for the wave function can be used to predict how the particles will behave under the influence of the specified potential and with each other.
Solution to equation
The Schrödinger equation is mathematically a wave equation, since the solutions are functions which describe wavelike motions. Normally wave equations in physics can be derived from other physical laws – the wave equation for mechanical vibrations on strings and in matter can be derived from Newton's laws – where the analogue wave function is the displacement of matter, and electromagnetic waves from Maxwell's equations, where the wave functions are electric and magnetic fields. On the contrary, the basis for Schrödinger's equation is the energy of the particle, and a separate postulate of quantum mechanics: the wave function is a description of the system.^{[23]} The SE is therefore a new concept in itself; as Feynman put it:
The particlelike behavior of the electron wave, for example, follows from the Schrödinger equation under the appropriate circumstances, as presented below. This behavior is often called waveparticle duality.
The Planck–Einstein and de Broglie relations
 $E=\backslash hbar\backslash omega,\; \backslash quad\; \backslash mathbf\{p\}=\backslash hbar\backslash mathbf\{k\}$
illuminate the deep connections between energy with time, and space with momentum. This is more apparent using natural units, setting ħ = 1 makes these equations into identities:
 $E=\backslash omega,\; \backslash quad\; \backslash mathbf\{p\}=\backslash mathbf\{k\}$
Energy and angular frequency both have the same dimensions as the reciprocal of time, and momentum and wavenumber both have the dimensions of inverse length. In practice they are used interchangeably; to prevent duplication of quantities and reduce the number of dimensions of related quantities. For familiarity SI units are still used in this article.
Schrödinger's insight, late in 1925, was to express the phase of a plane wave as a complex phase factor using these relations:
 $\backslash Psi\; =\; Ae^\{i(\backslash mathbf\{k\}\backslash cdot\backslash mathbf\{r\}\backslash omega\; t)\}\; =\; Ae^\{i(\backslash mathbf\{p\}\backslash cdot\backslash mathbf\{r\}Et)/\backslash hbar\}$
and to realize that the first order partial derivatives with respect to space
 $\backslash nabla\backslash Psi\; =\; \backslash dfrac\{i\}\{\backslash hbar\}\backslash mathbf\{p\}Ae^\{i(\backslash mathbf\{p\}\backslash cdot\backslash mathbf\{r\}Et)/\backslash hbar\}\; =\; \backslash dfrac\{i\}\{\backslash hbar\}\backslash mathbf\{p\}\backslash Psi$
and time
 $\backslash dfrac\{\backslash partial\; \backslash Psi\}\{\backslash partial\; t\}\; =\; \backslash dfrac\{i\; E\}\{\backslash hbar\}\; Ae^\{i(\backslash mathbf\{p\}\backslash cdot\backslash mathbf\{r\}Et)/\backslash hbar\}\; =\; \backslash dfrac\{i\; E\}\{\backslash hbar\}\; \backslash Psi$
imply the derivatives
 $\backslash begin\{matrix\}\; i\backslash hbar\backslash nabla\backslash Psi\; =\; \backslash mathbf\{p\}\backslash Psi\; \&\; \backslash rightarrow\; \&\; \backslash dfrac\{\backslash hbar^2\}\{2m\}\backslash nabla^2\backslash Psi\; =\; \backslash dfrac\{1\}\{2m\}\backslash mathbf\{p\}\backslash cdot\backslash mathbf\{p\}\backslash Psi\; \backslash \backslash $
\dfrac{\partial \Psi}{\partial t} = \dfrac{i E}{\hbar} \Psi & \rightarrow & i\hbar\dfrac{\partial \Psi}{\partial t} = E \Psi \\
\end{matrix}
Another postulate of quantum mechanics is that all observables are represented by operators which act on the wavefunction, and the eigenvalues of the operator are the values the observable takes. The previous derivatives lead to the energy operator, corresponding to the time derivative,
 $\backslash hat\{E\}=\; i\backslash hbar\backslash dfrac\{\backslash partial\}\{\backslash partial\; t\}$
and the momentum operator, corresponding to the spatial derivatives (the gradient),
 $\backslash hat\{\backslash mathbf\{p\}\}=\; i\backslash hbar\backslash nabla$
where the circumflexes ("hats") denote these observables are operators. These are differential operators, except for potential energy V which is just a multiplicative factor. An interesting point is that energy is also a symmetry with respect to time, and momentum is a symmetry with respect to space, and these are the reasons why energy and momentum are conserved – see Noether's theorem.
Multiplying the energy equation by Ψ, and substitution of the energy and momentum operators:
 $E=\; \backslash dfrac\{\backslash mathbf\{p\}\backslash cdot\backslash mathbf\{p\}\}\{2m\}+V\; \backslash rightarrow\; \backslash hat\{E\}\backslash Psi=\; \backslash dfrac\{\backslash hat\{\backslash mathbf\{p\}\}\backslash cdot\backslash hat\{\backslash mathbf\{p\}\}\}\{2m\}\backslash Psi+V\backslash Psi$
immediately led Schrödinger to his equation:
 $i\backslash hbar\backslash dfrac\{\backslash partial\; \backslash Psi\}\{\backslash partial\; t\}=\; \backslash dfrac\{\backslash hbar^2\}\{2m\}\backslash nabla^2\backslash Psi\; +V\backslash Psi$
These equations allow wave–particle duality can be assessed as follows. The kinetic energy T is related to the square of momentum p. As the particle's momentum increases, the kinetic energy increases more rapidly, but since the wavenumber k increases the wavelength $\backslash scriptstyle\; \backslash lambda$ decreases:
 $\backslash mathbf\{p\}\backslash cdot\backslash mathbf\{p\}\; \backslash propto\; \backslash mathbf\{k\}\backslash cdot\backslash mathbf\{k\}\; \backslash propto\; T\; \backslash propto\; \backslash dfrac\{1\}\{\backslash lambda^2\}$
and since the kinetic energy is also proportional to the second spatial derivatives, it is also proportional to the magnitude of the curvature of the wave:
 $\backslash hat\{T\}\; =\; \backslash frac\{\backslash hbar^2\}\{2m\}\backslash nabla\backslash cdot\backslash nabla\; \backslash propto\; \backslash nabla^2\; \backslash ,.$
As the curvature increases, the amplitude of the wave alternates between positive and negative more rapidly, and also shortens the wavelength. So the inverse relation between momentum and wavelength is consistent with the energy the particle has, and so the energy of the particle has a connection to a wave, all in the same mathematical formulation.^{[20]}
Solutions to equation
The general solutions of the equation can easily be seen as follows. The plane wave is definitely a solution because this was used to construct the equation, so due to linearity any linear combination of plane waves is also a solution. For discrete k the sum is a superposition of plane waves:
 $\backslash Psi(\backslash mathbf\{r\},t)\; =\; \backslash sum\_\{n=1\}^\backslash infty\; A\_n\; e^\{i(\backslash mathbf\{k\}\_n\backslash cdot\backslash mathbf\{r\}\backslash omega\_n\; t)\}\; \backslash ,\backslash !$
for some real amplitude coefficients A_{n}, and for continuous k the sum becomes an integral, the Fourier transform of a momentum space wavefunction:^{[24]}
 $\backslash Psi(\backslash mathbf\{r\},t)\; =\; \backslash frac\{1\}\{(\backslash sqrt\{2\backslash pi\})^3\}\backslash int\backslash Phi(\backslash mathbf\{k\})e^\{i(\backslash mathbf\{k\}\backslash cdot\backslash mathbf\{r\}\backslash omega\; t)\}d^3\backslash mathbf\{k\}\; \backslash ,\backslash !$
where d^{3}k = dk_{x}dk_{y}dk_{z} is the differential volume element in kspace, and the integrals are taken over all kspace. The momentum wavefunction Φ(k) arises in the integrand since the position and momentum space wavefunctions are Fourier transforms of each other. Since these satisfy the Schrödinger equation—the solutions to Schrödinger's equation for a given situation will not only be the plane waves used to obtain it, but any wavefunctions which satisfy the Schrödinger's equation prescribed by the system, in addition to the relevant boundary conditions—it can be concluded the Schrödinger equation is true for any (nonrelativistic) situation.
To summarize, the Schrödinger equation is a differential equation of wave–particle duality, and particles can behave like waves because their corresponding wavefunction satisfies the equation.
Wave and particle motion
Increasing levels of
wavepacket localization, meaning the particle has a more localized position.
In the limit ħ → 0, the particle's position and momentum become known exactly. This is equivalent to the classical particle.
Schrödinger required that a wave packet solution near position r with wavevector near k will move along the trajectory determined by classical mechanics for times short enough for the spread in k (and hence in velocity) not to substantially increase the spread in r . Since, for a given spread in k, the spread in velocity is proportional to Planck's constant ħ, it is sometimes said that in the limit as ħ approaches zero, the equations of classical mechanics are restored from quantum mechanics.^{[25]} Great care is required in how that limit is taken, and in what cases.
The limiting shortwavelength is equivalent to ħ tending to zero because this is limiting case of increasing the wave packet localization to the definite position of the particle (see images right). Using the Heisenberg uncertainty principle for position and momentum, the products of uncertainty in position and momentum become zero as ħ → 0:
 $\backslash sigma(x)\; \backslash sigma(p\_x)\; \backslash geqslant\; \backslash frac\{\backslash hbar\}\{2\}\; \backslash quad\; \backslash rightarrow\; \backslash quad\; \backslash sigma(x)\; \backslash sigma(p\_x)\; \backslash geqslant\; 0\; \backslash ,\backslash !$
where σ denotes the (root mean square) measurement uncertainty in x and p_{x} (and similarly for the y and z directions) which implies the position and momentum can only be known to arbitrary precision in this limit.
The Schrödinger equation in its general form
 $i\backslash hbar\; \backslash frac\{\backslash partial\}\{\backslash partial\; t\}\; \backslash Psi\backslash left(\backslash mathbf\{r\},t\backslash right)\; =\; \backslash hat\{H\}\; \backslash Psi\backslash left(\backslash mathbf\{r\},t\backslash right)\; \backslash ,\backslash !$
is closely related to the Hamilton–Jacobi equation (HJE)
 $\backslash frac\{\backslash partial\}\{\backslash partial\; t\}\; S(q\_i,t)\; =\; H\backslash left(q\_i,\backslash frac\{\backslash partial\; S\}\{\backslash partial\; q\_i\},t\; \backslash right)\; \backslash ,\backslash !$
where S is action and H is the Hamiltonian function (not operator). Here the generalized coordinates q_{i} for i = 1, 2, 3 (used in the context of the HJE) can be set to the position in Cartesian coordinates as r = (q_{1}, q_{2}, q_{3}) = (x, y, z).^{[25]}
Substituting
 $\backslash Psi\; =\; \backslash sqrt\{\backslash rho(\backslash mathbf\{r\},t)\}\; e^\{iS(\backslash mathbf\{r\},t)/\backslash hbar\}\backslash ,\backslash !$
where ρ is the probability density, into the Schrödinger equation and then taking the limit ħ → 0 in the resulting equation, yields the Hamilton–Jacobi equation.
The implications are:
 The motion of a particle, described by a (shortwavelength) wave packet solution to the Schrödinger equation, is also described by the Hamilton–Jacobi equation of motion.
 The Schrödinger equation includes the wavefunction, so its wave packet solution implies the position of a (quantum) particle is fuzzily spread out in wave fronts. On the contrary, the Hamilton–Jacobi equation applies to a (classical) particle of definite position and momentum, instead the position and momentum at all times (the trajectory) are deterministic and can be simultaneously known.
Nonrelativistic quantum mechanics
The quantum mechanics of particles propagating at speeds much less than light is known as nonrelativistic quantum mechanics. Following are several forms of Schrödinger's equation in this context for different situations: time independence and dependence, one and three spatial dimensions, and one and N particles. In actuality, the particles constituting the system do not have the numerical labels used in theory. The language of mathematics forces us to label the positions of particles one way or another, otherwise there would be confusion between symbols representing which variables are for which particle.^{[22]}
Time independent
If the Hamiltonian is not an explicit function of time, the equation is separable into its spatial and temporal parts, i.e. $\backslash Psi(x,t)=\backslash psi(x)\backslash tau(t)$. Hence the energy operator $\backslash hat\{E\}\; =\; i\; \backslash hbar\; \backslash partial\; /\; \backslash partial\; t\; \backslash ,\backslash !$ can then be replaced by the energy eigenvalue E. In abstract form, it is an eigenvalue equation for the Hamiltonian $\backslash hat\{H\}$ ^{[26]}
 $\backslash hat\; H\; \backslash psi\; =\; E\; \backslash psi$
A solution of the timeindependent equation is called an energy eigenstate with energy E.^{[24]}
To find the time dependence of the state, consider starting the timedependent equation with an initial condition ψ(r). The time derivative at t = 0 is everywhere proportional to the value:
 $\backslash left.i\backslash hbar\; \backslash frac\{\backslash partial\}\{\backslash partial\; t\}\; \backslash Psi(\backslash mathbf\{r\},t)\backslash right\_\{t=0\}=\; \backslash left.H\; \backslash Psi(\backslash mathbf\{r\},t)\backslash right\_\{t=0\}\; =E\; \backslash Psi(\backslash mathbf\{r\},0)\; \backslash ,$
So initially the whole function just gets rescaled, and it maintains the property that its time derivative is proportional to itself, so for all times t,
 $\backslash Psi(\backslash mathbf\{r\},t)=\; \backslash tau(t)\; \backslash psi(\backslash mathbf\{r\})\; \backslash ,$
substituting for Ψ:
 $i\backslash hbar\; \backslash frac\{\backslash partial\; \backslash Psi\}\{\backslash partial\; t\; \}\; =\; E\; \backslash Psi\; \backslash rightarrow\; i\backslash hbar\; \backslash psi(\backslash mathbf\{r\})\backslash frac\{\backslash partial\backslash tau(t)\}\{\backslash partial\; t\; \}\; =\; E\; \backslash tau(t)\backslash psi(\backslash mathbf\{r\})\; \backslash ,$
where the ψ(r) cancels, so solving this equation for $\backslash scriptstyle\; \backslash tau(t)\backslash ,\backslash !$ implies the solution of the timedependent equation with this initial condition is:^{[11]}
 $\backslash Psi(\backslash mathbf\{r\},t)\; =\; \backslash psi(\backslash mathbf\{r\})\; e^\{i\{E\; t/\backslash hbar\}\}\; =\; \backslash psi(\backslash mathbf\{r\})\; e^\{i\{\backslash omega\; t\}\}\; \backslash ,$
This case describes the standing wave solutions of the timedependent equation, which are the states with definite energy (instead of a probability distribution of different energies). In physics, these standing waves are called "stationary states" or "energy eigenstates"; in chemistry they are called "atomic orbitals" or "molecular orbitals". Superpositions of energy eigenstates change their properties according to the relative phases between the energy levels.
The energy eigenvalues from this equation form a discrete spectrum of values, so mathematically energy must be quantized. More specifically, the energy eigenstates form a basis – any wavefunction may be written as a sum over the discrete energy states or an integral over continuous energy states, or more generally as an integral over a measure. This is the spectral theorem in mathematics, and in a finite state space it is just a statement of the completeness of the eigenvectors of a Hermitian matrix.
In the case of atoms and molecules, it turns out in spectroscopy that the discrete spectral lines of atoms is evidence that energy is indeed physically quantized in atoms; specifically there are energy levels in atoms, associated with the atomic or molecular orbitals of the electrons (the stationary states, wavefunctions). The spectral lines observed are definite frequencies of light, corresponding to definite energies, by the Planck–Einstein relation and De Broglie relations (above). However, it is not the absolute value of the energy level, but the difference between them, which produces the observed frequencies, due to electronic transitions within the atom emitting/absorbing photons of light.
Onedimensional examples
For a particle in one dimension, the Hamiltonian is:
 $\backslash hat\{H\}\; =\; \backslash frac\{\backslash hat\{p\}^2\}\{2m\}\; +\; V(x)\; =\; \backslash frac\{\backslash hbar^2\}\{2m\}\backslash frac\{d^2\}\{d\; x^2\}\; +\; V(x)$
and substituting this into the general Schrödinger equation gives:
 $E\backslash Psi\; =\; \backslash frac\{\backslash hbar^2\}\{2m\}\backslash frac\{d^2\}\{d\; x^2\}\backslash Psi\; +\; V\backslash Psi$
This is the only case in which the equation is an ordinary differential equation, rather than a partial differential equation: in higher spatial dimensions and for particles, more spatial derivatives occur for each particle, as shown below. The solutions is always of the form:
 $\backslash Psi(x,t)=\backslash psi(x)\; e^\{iEt/\backslash hbar\}\; \backslash ,\; .$
There is a further restriction — the solution must not grow at infinity, so that it has either a finite L^{2}norm (if it is a bound state) or a slowly diverging norm (if it is part of a continuum):^{[27]}
 $\backslash \; \backslash psi\; \backslash ^2\; =\; \backslash int\; \backslash psi(x)^2\backslash ,\; dx.\backslash ,$
For N particles in one dimension, the Hamiltonian is:
 $\backslash begin\{align\}\backslash hat\{H\}\; \&=\; \backslash sum\_\{n=1\}^\{N\}\backslash frac\{\backslash hat\{p\}\_n^2\}\{2m\_n\}\; +\; V(x\_1,x\_2,\backslash cdots\; x\_N)\; \backslash \backslash $
& = \frac{\hbar^2}{2}\sum_{n=1}^{N}\frac{1}{m_n}\frac{\partial^2}{\partial x_n^2} + V(x_1,x_2,\cdots x_N)
\end{align}
where the position of particle n is x_{n}. The corresponding Schrödinger equation is:
 $E\backslash Psi\; =\; \backslash frac\{\backslash hbar^2\}\{2\}\backslash sum\_\{n=1\}^\{N\}\backslash frac\{1\}\{m\_n\}\backslash frac\{\backslash partial^2\}\{\backslash partial\; x\_n^2\}\backslash Psi\; +\; V\backslash Psi\; \backslash ,\; .$
with solutions of the form:
 $\backslash Psi\; =\; e^\{iEt/\backslash hbar\}\backslash psi(x\_1,x\_2\backslash cdots\; x\_N)$
For noninteracting particles, the potential of the system only influences each particle separately, so the total potential energy is the sum of potential energies for each particle:
 $V(x\_1,x\_2,\backslash cdots\; x\_N)\; =\; \backslash sum\_\{n=1\}^N\; V(x\_n)\; \backslash ,\; .$
and the wavefunction can be written as a product of the wavefunctions for each particle:
 $\backslash Psi\; =\; e^\{i\{E\; t/\backslash hbar\}\}\backslash prod\_\{n=1\}^N\backslash psi(x\_n)\; \backslash ,\; ,$
In general for interacting particles, the above decompositions are not possible.
Free particle
For no potential, V = 0, so the particle is free and the equation reads:^{[28]}
 $\; E\; \backslash psi\; =\; \backslash frac\{\backslash hbar^2\}\{2m\}\{d^2\; \backslash psi\; \backslash over\; d\; x^2\}\backslash ,$
which has oscillatory solutions for E > 0 (the C_{n} are arbitrary constants):
 $\backslash psi\_E(x)\; =\; C\_1\; e^\{i\backslash sqrt\{2mE/\backslash hbar^2\}\backslash ,x\}\; +\; C\_2\; e^\{i\backslash sqrt\{2mE/\backslash hbar^2\}\backslash ,x\}\backslash ,$
and exponential solutions for E < 0
 $\backslash psi\_\{E\}(x)\; =\; C\_1\; e^\{\backslash sqrt\{2mE/\backslash hbar^2\}\backslash ,x\}\; +\; C\_2\; e^\{\backslash sqrt\{2mE/\backslash hbar^2\}\backslash ,x\}.\backslash ,$
The exponentially growing solutions have an infinite norm, and are not physical. They are not allowed in a finite volume with periodic or fixed boundary conditions.
Constant potential
For a constant potential, V = V_{0}, the solution is oscillatory for E > V_{0} and exponential for E < V_{0}, corresponding to energies that are allowed or disallowed in classical mechanics. Oscillatory solutions have a classically allowed energy and correspond to actual classical motions, while the exponential solutions have a disallowed energy and describe a small amount of quantum bleeding into the classically disallowed region, due to quantum tunneling. If the potential V_{0} grows at infinity, the motion is classically confined to a finite region, which means that in quantum mechanics every solution becomes an exponential far enough away. The condition that the exponential is decreasing restricts the energy levels to a discrete set, called the allowed energies.^{[24]}
Harmonic oscillator
The Schrödinger equation for this situation is
 $E\backslash psi\; =\; \backslash frac\{\backslash hbar^2\}\{2m\}\backslash frac\{d^2\}\{d\; x^2\}\backslash psi\; +\; \backslash frac\{1\}\{2\}m\backslash omega^2x^2\backslash psi$
It is a notable quantum system to solve for; since the solutions are exact (but complicated – in terms of Hermite polynomials), and it can describe or at least approximate a wide variety of other systems, including vibrating atoms, molecules,^{[29]} and atoms or ions in lattices,^{[30]} and approximating other potentials near equilibrium points. It is also the basis of perturbation methods in quantum mechanics.
There is a family of solutions – in the position basis they are
 $\backslash psi\_n(x)\; =\; \backslash sqrt\{\backslash frac\{1\}\{2^n\backslash ,n!\}\}\; \backslash cdot\; \backslash left(\backslash frac\{m\backslash omega\}\{\backslash pi\; \backslash hbar\}\backslash right)^\{1/4\}\; \backslash cdot\; e^\{$
 \frac{m\omega x^2}{2 \hbar}} \cdot H_n\left(\sqrt{\frac{m\omega}{\hbar}} x \right)
where n = 0,1,2..., and the functions H_{n} are the Hermite polynomials.
Threedimensional examples
The extension from one dimension to three dimensions is straightforward, all position and momentum operators are replaced by their threedimensional expressions and the partial derivative with respect to space is replaced by the gradient operator.
The Hamiltonian for one particle in three dimensions is:
 $\backslash begin\{align\}\backslash hat\{H\}\; \&\; =\; \backslash frac\{\backslash hat\{\backslash mathbf\{p\}\}\backslash cdot\backslash hat\{\backslash mathbf\{p\}\}\}\{2m\}\; +\; V(\backslash mathbf\{r\})\; \backslash \backslash $
& = \frac{\hbar^2}{2m}\nabla^2 + V(\mathbf{r})
\end{align}
generating the equation:
 $E\backslash Psi\; =\; \backslash frac\{\backslash hbar^2\}\{2m\}\backslash nabla^2\backslash Psi\; +\; V\backslash Psi$
with stationary state solutions of the form:
 $\backslash Psi\; =\; \backslash psi(\backslash mathbf\{r\})\; e^\{iEt/\backslash hbar\}$
where the position of the particle is r = (x, y, z).
For N particles in three dimensions, the Hamiltonian is:
 $\backslash begin\{align\}\; \backslash hat\{H\}\; \&\; =\; \backslash sum\_\{n=1\}^\{N\}\backslash frac\{\backslash hat\{\backslash mathbf\{p\}\}\_n\backslash cdot\backslash hat\{\backslash mathbf\{p\}\}\_n\}\{2m\_n\}\; +\; V(\backslash mathbf\{r\}\_1,\backslash mathbf\{r\}\_2,\backslash cdots\backslash mathbf\{r\}\_N)\; \backslash \backslash $
& = \frac{\hbar^2}{2}\sum_{n=1}^{N}\frac{1}{m_n}\nabla_n^2 + V(\mathbf{r}_1,\mathbf{r}_2,\cdots\mathbf{r}_N)
\end{align}
where the position of particle n is r_{n} = (x_{n}, y_{n}, z_{n}), and the gradient operators are partial derivatives with respect to the particle's position coordinates. In Cartesian coordinates, for particle n:
 $\backslash nabla\_n\; =\; \backslash mathbf\{e\}\_x\; \backslash frac\{\backslash partial\}\{\backslash partial\; x\_n\}\; +\; \backslash mathbf\{e\}\_y\backslash frac\{\backslash partial\}\{\backslash partial\; y\_n\}\; +\; \backslash mathbf\{e\}\_z\backslash frac\{\backslash partial\}\{\backslash partial\; z\_n\}$
with corresponding Laplacian operator for particle n:
 $\backslash nabla\_n^2\; =\; \backslash nabla\_n\backslash cdot\backslash nabla\_n\; =\; \backslash frac\{\backslash partial^2\}\backslash prod\_\{n=1\}^N\backslash psi(\backslash mathbf\{r\}\_n)\; \backslash ,\; ,\; \backslash quad\; V(\backslash mathbf\{r\}\_1,\backslash mathbf\{r\}\_2,\backslash cdots\; \backslash mathbf\{r\}\_N)\; =\; \backslash sum\_\{n=1\}^N\; V(\backslash mathbf\{r\}\_n)$
and this does not apply to interacting particles.
Following are examples where exact solutions are known. See the main articles for further details.
Hydrogen atom
This form of the Schrödinger equation can be applied to the hydrogen atom:^{[20]}^{[23]}
 $E\; \backslash psi\; =\; \backslash frac\{\backslash hbar^2\}\{2\backslash mu\}\backslash nabla^2\backslash psi\; \; \backslash frac\{e^2\}\{4\backslash pi\backslash epsilon\_0\; r\}\backslash psi$
where e is the electron charge, r is the position of the electron (r = is the magnitude of the position), the potential term is due to the coloumb interaction, wherein ε_{0} is the electric constant (permittivity of free space) and
 $\backslash mu\; =\; \backslash frac\{m\_em\_p\}\{m\_e+m\_p\}$
is the 2body reduced mass of the hydrogen nucleus (just a proton) of mass m_{p} and the electron of mass m_{e}. The negative sign arises in the potential term since the proton and electron are oppositely charged. The reduced mass in place of the electron mass is used since the electron and proton together orbit each other about a common centre of mass, and constitute a twobody problem to solve. The motion of the electron is of principle interest here, so the equivalent onebody problem is the motion of the electron using the reduced mass.
The wavefunction for hydrogen is a function of the electron's coordinates, and in fact can be separated into functions of each coordinate.^{[31]} Usually this is done in spherical polar coordinates:
 $\backslash psi(r,\backslash theta,\backslash phi)\; =\; R(r)Y\_\backslash ell^m(\backslash theta,\; \backslash phi)\; =\; R(r)\backslash Theta(\backslash theta)\backslash Phi(\backslash phi)$
where R are radial functions and $\backslash scriptstyle\; Y\_\{\backslash ell\}^\{m\}(\backslash theta,\; \backslash phi\; )\; \backslash ,$ are spherical harmonics of degree ℓ and order m. This is the only atom for which the Schrödinger equation has been solved for exactly. Multielectron atoms require approximative methods. The family of solutions are:^{[32]}
 $\backslash psi\_\{n\backslash ell\; m\}(r,\backslash theta,\backslash phi)\; =\; \backslash sqrt\; \}\; e^\{\; r/na\_0\}\; \backslash left(\backslash frac\{2r\}\{na\_0\}\backslash right)^\{\backslash ell\}\; L\_\{n\backslash ell1\}^\{2\backslash ell+1\}\backslash left(\backslash frac\{2r\}\{na\_0\}\backslash right)\; \backslash cdot\; Y\_\{\backslash ell\}^\{m\}(\backslash theta,\; \backslash phi\; )$
where:
 $\backslash begin\{align\}\; n\; \&\; =\; 1,2,3\; \backslash cdots\; \backslash \backslash $
\ell & = 0,1,2 \cdots n1 \\
m & = \ell\cdots\ell
\end{align}
NB: generalized Laguerre polynomials are defined differently by different authors—see main article on them and the hydrogen atom.
Twoelectron atoms or ions
The equation for any twoelectron system, such as the neutral helium atom (He, Z = 2), the negative hydrogen ion (H^{–}, Z = 1), or the positive lithium ion (Li^{+}, Z = 3) is:^{[21]}
 $E\backslash psi\; =\; \backslash hbar^2\backslash left[\backslash frac\{1\}\{2\backslash mu\}\backslash left(\backslash nabla\_1^2\; +\backslash nabla\_2^2\; \backslash right)\; +\; \backslash frac\{1\}\{M\}\backslash nabla\_1\backslash cdot\backslash nabla\_2\backslash right]\; \backslash psi\; +\; \backslash frac\{e^2\}\{4\backslash pi\backslash epsilon\_0\}\backslash left[\; \backslash frac\{1\}\{r\_\{12\}\}\; Z\backslash left(\; \backslash frac\{1\}\{r\_1\}+\backslash frac\{1\}\{r\_2\}\; \backslash right)\; \backslash right]\; \backslash psi$
where r_{1} is the position of one electron (r_{1} = is its magnitude), r_{2} is the position of the other electron (r_{2} = r_{2} is the magnitude), r_{12} = r_{12} is the magnitude of the separation between them given by
 $\backslash mathbf\{r\}\_\{12\}\; =\; \backslash mathbf\{r\}\_2\; \; \backslash mathbf\{r\}\_1\; \; \backslash ,\backslash !$
μ is again the twobody reduced mass of an electron with respect to the nucleus of mass M, so this time
 $\backslash mu\; =\; \backslash frac\{m\_e\; M\}\{m\_e+M\}\; \backslash ,\backslash !$
and Z is the atomic number for the element (not a quantum number).
The crossterm of two laplacians
 $\backslash frac\{1\}\{M\}\backslash nabla\_1\backslash cdot\backslash nabla\_2\backslash ,\backslash !$
is known as the mass polarization term, which arises due to the motion of atomic nuclei. The wavefunction is a function of the two electron's positions:
 $\backslash psi\; =\; \backslash psi(\backslash mathbf\{r\}\_1,\backslash mathbf\{r\}\_2).$
There is no closed form solution for this equation.
Time dependent
This is the equation of motion for the quantum state. In the most general form, it is written:^{[26]}
 $i\; \backslash hbar\; \backslash frac\{\backslash partial\}\{\backslash partial\; t\}\backslash Psi\; =\; \backslash hat\; H\; \backslash Psi.$
For one particle in one dimension:
 $\backslash hat\{H\}\; =\; \backslash frac\{\backslash hat\{p\}^2\}\{2m\}\; +\; V(x,t)\; =\; \backslash frac\{\backslash hbar^2\}\{2m\}\backslash frac\{\backslash partial^2\}\{\backslash partial\; x^2\}\; +\; V(x,t)$
generating the equation:
 $i\backslash hbar\backslash frac\{\backslash partial\}\{\backslash partial\; t\}\backslash Psi\; =\; \backslash frac\{\backslash hbar^2\}\{2m\}\backslash frac\{\backslash partial^2\}\{\backslash partial\; x^2\}\backslash Psi\; +\; V\backslash Psi$
and solutions:
 $\backslash Psi\; =\; \backslash Psi(x,t)$
For N particles in one dimension, the Hamiltonian is:
 $\backslash begin\{align\}\backslash hat\{H\}\; \&=\; \backslash sum\_\{n=1\}^\{N\}\backslash frac\{\backslash hat\{p\}\_n^2\}\{2m\_n\}\; +\; V(x\_1,x\_2,\backslash cdots\; x\_N,t)\; \backslash \backslash $
& = \frac{\hbar^2}{2}\sum_{n=1}^{N}\frac{1}{m_n}\frac{\partial^2}{\partial x_n^2} + V(x_1,x_2,\cdots x_N,t)
\end{align}
where the position of particle n is x_{n}, generating the equation:
 $i\backslash hbar\backslash frac\{\backslash partial\}\{\backslash partial\; t\}\backslash Psi\; =\; \backslash frac\{\backslash hbar^2\}\{2\}\backslash sum\_\{n=1\}^\{N\}\backslash frac\{1\}\{m\_n\}\backslash frac\{\backslash partial^2\}\{\backslash partial\; x\_n^2\}\backslash Psi\; +\; V\backslash Psi\; \backslash ,\; .$
with solutions:
 $\backslash Psi\; =\; \backslash Psi(x\_1,x\_2\backslash cdots\; x\_N,t)$
For one particle in three dimensions, the Hamiltonian is:
 $\backslash begin\{align\}\backslash hat\{H\}\; \&\; =\; \backslash frac\{\backslash hat\{\backslash mathbf\{p\}\}\backslash cdot\backslash hat\{\backslash mathbf\{p\}\}\}\{2m\}\; +\; V(\backslash mathbf\{r\},t)\; \backslash \backslash $
& = \frac{\hbar^2}{2m}\nabla^2 + V(\mathbf{r},t) \\
\end{align}
generating the equation:
 $i\backslash hbar\backslash frac\{\backslash partial\}\{\backslash partial\; t\}\backslash Psi\; =\; \backslash frac\{\backslash hbar^2\}\{2m\}\backslash nabla^2\backslash Psi\; +\; V\backslash Psi$
with solutions:
 $\backslash Psi\; =\; \backslash Psi(\backslash mathbf\{r\},t)$
For N particles in three dimensions, the Hamiltonian is:
 $\backslash begin\{align\}\; \backslash hat\{H\}\; \&\; =\; \backslash sum\_\{n=1\}^\{N\}\backslash frac\{\backslash hat\{\backslash mathbf\{p\}\}\_n\backslash cdot\backslash hat\{\backslash mathbf\{p\}\}\_n\}\{2m\_n\}\; +\; V(\backslash mathbf\{r\}\_1,\backslash mathbf\{r\}\_2,\backslash cdots\backslash mathbf\{r\}\_N,t)\; \backslash \backslash $
& = \frac{\hbar^2}{2}\sum_{n=1}^{N}\frac{1}{m_n}\nabla_n^2 + V(\mathbf{r}_1,\mathbf{r}_2,\cdots\mathbf{r}_N,t)
\end{align}
generating the equation:
 $i\backslash hbar\backslash frac\{\backslash partial\}\{\backslash partial\; t\}\backslash Psi\; =\; \backslash frac\{\backslash hbar^2\}\{2\}\backslash sum\_\{n=1\}^\{N\}\backslash frac\{1\}\{m\_n\}\backslash nabla\_n^2\backslash Psi\; +\; V\backslash Psi$
This last equation is in a very high dimension,^{[33]} so the solutions
 $\backslash Psi\; =\; \backslash Psi(\backslash mathbf\{r\}\_1,\backslash mathbf\{r\}\_2,\backslash cdots\backslash mathbf\{r\}\_N,t)$
are not easy to visualize.
Solution methods

Methods for special cases:

Properties
The Schrödinger equation has the following properties: some are useful, but there are shortcomings. Ultimately, these properties arise from the Hamiltonian used, and solutions to the equation.
Linearity
In the development above, the Schrödinger equation was made to be linear for generality, though this has other implications. If two wave functions ψ_{1} and ψ_{2} are solutions, then so is any linear combination of the two:
 $\backslash displaystyle\; \backslash psi\; =\; a\backslash psi\_1\; +\; b\; \backslash psi\_2$
where a and b are any complex numbers (the sum can be extended for any number of wavefunctions). This property allows superpositions of quantum states to be solutions of the Schrödinger equation. Even more generally, it holds that a general solution to the Schrödinger equation can be found by taking a weighted sum over all single state solutions achievable. For example, consider a wave function $\backslash Psi\; (x,t)$ such that the wave function is a product of two functions: one time independent, and one time dependent. If states of definite energy found using the time independent Shrödinger equation are given by $\backslash psi\_E\; (x)$ with amplitude $A\_n$ and time dependent phase factor is given by
 $e^\backslash Psi\; \; \backslash Psi\backslash hat\{\backslash mathbf\{p\}\}\backslash Psi^*\; \backslash right)\backslash ,\backslash !$
is the probability current (flow per unit area).
Hence predictions from the Schrödinger equation do not violate probability conservation. However, the continuity equation is more fundamental and intuitive than the SE itself, and is always true, while SE is not.
Positive energy
If the potential is bounded from below, meaning there is a minimum value of potential energy, the eigenfunctions of the Schrödinger equation have energy which is also bounded from below. This can be seen most easily by using the variational principle, as follows. (See also below).
For any linear operator $\backslash hat\{A\}$ bounded from below, the eigenvector with the smallest eigenvalue is the vector ψ that minimizes the quantity
 $\backslash langle\; \backslash psi\; \backslash hat\{A\}\backslash psi\; \backslash rangle$
over all ψ which are normalized.^{[34]} In this way, the smallest eigenvalue is expressed through the variational principle. For the Schrödinger Hamiltonian $\backslash hat\{H\}$ bounded from below, the smallest eigenvalue is called the ground state energy. That energy is the minimum value of
 $\backslash langle\; \backslash psi\backslash hat\{H\}\backslash psi\backslash rangle\; =\; \backslash int\; \backslash psi^*(\backslash mathbf\{r\})\; \backslash left[\; \; \backslash frac\{\backslash hbar^2\}\{2m\}\; \backslash nabla^2\backslash psi(\backslash mathbf\{r\})\; +\; V(\backslash mathbf\{r\})\backslash psi(\backslash mathbf\{r\})\backslash right]\; d^3\backslash mathbf\{r\}\; =\; \backslash int\; \backslash left[\; \backslash frac\{\backslash hbar^2\}\{2m\}\backslash nabla\backslash psi^2\; +\; V(\backslash mathbf\{r\})\; \backslash psi^2\; \backslash right]\; d^3\backslash mathbf\{r\}\; =\; \backslash langle\; \backslash hat\{H\}\backslash rangle$
(using integration by parts). Due to the complex modulus of ψ squared (which is positive definite), the right hand side always greater than the lowest value of V(x). In particular, the ground state energy is positive when V(x) is everywhere positive.
For potentials which are bounded below and are not infinite over a region, there is a ground state which minimizes the integral above. This lowest energy wavefunction is real and positive definite – meaning the wavefunction can increase and decrease, but is positive for all positions. It physically cannot be negative: if it were, smoothing out the bends at the sign change (to minimize the wavefunction) rapidly reduces the gradient contribution to the integral and hence the kinetic energy, while the potential energy changes linearly and less quickly. The kinetic and potential energy are both changing at different rates, so the total energy is not constant, which can't happen (conservation). The solutions are consistent with Schrödinger equation if this wavefunction is positive definite.
The lack of sign changes also shows that the ground state is nondegenerate, since if there were two ground states with common energy E, not proportional to each other, there would be a linear combination of the two that would also be a ground state resulting in a zero solution.
Analytic continuation to diffusion
The above properties (positive definiteness of energy) allow the analytic continuation of the Schrödinger equation to be identified as a stochastic process. This can be interpreted as the Huygens–Fresnel principle applied to De Broglie waves; the spreading wavefronts are diffusive probability amplitudes.^{[34]}
For a particle in a random walk (again for which V = 0), the continuation is to let:^{[35]}
 $\backslash tau\; =\; it\; \backslash ,,\; \backslash !$
substituting into the timedependent Schrödinger equation gives:
 $\{\backslash partial\; \backslash over\; \backslash partial\; \backslash tau\}\; \backslash Psi(\backslash mathbf\{r\},\backslash tau)\; =\; \backslash frac\{\backslash hbar\}\{2m\}\; \backslash nabla\; ^2\; \backslash Psi(\backslash mathbf\{r\},\backslash tau),$
which has the same form as the diffusion equation, with diffusion coefficient ħ/2m.
Galilean and Lorentz transformations
Nonrelativistic
The solutions to the Schrödinger equation are not Galilean invariant, so the equation itself is not either, as outlined below. Changing inertial reference frames requires a transformation of the wavefunction analogous to requiring gauge invariance. This transformation introduces a phase factor that is normally ignored as nonphysical, but has application in some problems.^{[36]}
Galilean transformations (or "boosts") look at the system from the point of view of an observer moving with a steady velocity –v. A boost must change the physical properties of a wavepacket in the same way as in classical mechanics:^{[34]}
 $\backslash mathbf\{r\}\text{'}=\; \backslash mathbf\{r\}\; +\; \backslash mathbf\{v\}t\; \backslash quad\; \backslash mathbf\{p\}\text{'}=\; \backslash mathbf\{p\}\; +\; m\backslash mathbf\{v\}.\; \backslash ,$
For a freeparticle solution (plane wave), Galilean invariance requires the phase (complex exponent) to remain in the same form:
 $\backslash Psi\; =\; Ae^\{i(\backslash mathbf\{p\}\backslash cdot\backslash mathbf\{r\}\; \; E\; t)/\backslash hbar\}\; =\; Ae^\{i(\backslash mathbf\{p\}\text{'}\backslash cdot\backslash mathbf\{r\}\text{'}\; \; E\text{'}\; t)/\backslash hbar\}\; \backslash ,\backslash !$
but instead the phase terms transform according to
 $\backslash begin\{align\}\; (\backslash mathbf\{p\}\backslash cdot\backslash mathbf\{r\}\; \; E\; t)/\backslash hbar\; \&\; \backslash rightarrow\; \backslash left[(\backslash mathbf\{p\}\text{'}\; \; m\backslash mathbf\{v\})\backslash cdot(\backslash mathbf\{r\}\text{'}\; \; \backslash mathbf\{v\}t)\; \; (\backslash mathbf\{p\}\text{'}m\backslash mathbf\{v\})\backslash cdot(\backslash mathbf\{p\}\text{'}m\backslash mathbf\{v\})t/2m\; \backslash right]/\backslash hbar\; \backslash \backslash $
& = \left[(\mathbf{p}' \cdot\mathbf{r}' + E' t) + (\mathbf{p} \cdot \mathbf{r}  Et) \right]/\hbar \\
\end{align}
(energy is kinetic energy, since V = 0) therefore the plane wave transforms by
 $\backslash begin\{align\}$
\Psi = Ae^{i(\mathbf{p}\cdot\mathbf{r}  E t)/\hbar} & \rightarrow & \Psi' & = Ae^{i(\mathbf{p}'\cdot\mathbf{r}'  E' t)/\hbar} e^{i(\mathbf{p} \cdot \mathbf{r}  Et)/\hbar} \\
& & & = \Psi e^{i(\mathbf{p}'\cdot\mathbf{r}'  E' t)/\hbar} \\
& & & \neq Ae^{i(\mathbf{p}'\cdot\mathbf{r}'  E' t)/\hbar}. \\
\end{align}\,\!
So by contradiction, the extra phase factor implies the Schrödinger equation is not Galilean invariant: the plane wave solutions do not remain in the same form under Galilean transformations. A linear combination of plane waves, with different values of p and E, will also transform in the same way due to linearity. In general the transformation of any solution to the freeparticle Schrödinger equation, Ψ(r, t) results in other "boosted" solutions:
 $\backslash Psi\text{'}(\backslash mathbf\{r\}\text{'},t)\; =\; \backslash Psi(\backslash mathbf\{r\},t)\; e^\{\; i\; (\backslash mathbf\{p\}\text{'}\; \backslash cdot\; \backslash mathbf\{r\}\text{'}\; \; E\text{'}t)/\backslash hbar\}\; =\; \backslash Psi(\backslash mathbf\{r\}\text{'}\; \; \backslash mathbf\{v\}t,t)\; e^\{\; i\; (\backslash mathbf\{p\}\text{'}\; \backslash cdot\; \backslash mathbf\{r\}\text{'}\; \; E\text{'}t)/\backslash hbar\}.\; \backslash ,$
Relativistic
The Lorentz transformations are (slightly) more complicated than the Galilean ones, so the solutions to the Schrödinger equation are certainly not Lorentz invariant either, in turn not consistent with special relativity. Also, as shown above in the plausibility argument – the Schrödinger equation was constructed from classical energy conservation rather than the relativistic energy–momentum relation
 $E^2\; =\; (pc)^2\; +\; (m\_0c^2)^2\; \backslash ,\; .$
This relativistic equation is Lorentz invariant. The classical equation is not – it is the lowvelocity limit of the relativistic equation (velocities much less than the speed of light). This further shows that the Schrödinger equation itself, not just the solutions, is not Lorentz invariant.
Secondly, the equation requires the particles to be the same type, and the number of particles in the system to be constant, since their masses are constants in the equation (kinetic energy terms).^{[37]} This alone means the Schrödinger equation is not compatible with relativity – even the simple equation
 $\backslash displaystyle\; E\; =\; mc^2$
allows (in highenergy processes) particles of matter to completely transform into energy by particle–antiparticle annihilation, and enough energy can recreate other particle–antiparticle pairs. So the number of particles and types of particles is not necessarily fixed. For all other intrinsic properties of the particles which may enter the potential function, including mass (such as the harmonic oscillator) and charge (such as electrons in atoms), which will also be constants in the equation, the same problem follows.
Relativistic quantum mechanics
Relativistic quantum mechanics is obtained where quantum mechanics and special relativity simultaneously apply. The general form of the Schrödinger equation is still applicable, but the Hamiltonian operators are much less obvious, and more complicated.
One wishes to build relativistic wave equations from the relativistic energy–momentum relation. The Klein–Gordon equation and the Dirac equation are two such equations. The Klein–Gordon equation was the first such equation to be obtained, even before the nonrelativistic one, and applies to massive spinless particles. The Dirac equation arose from taking the "square root" of the Klein–Gordon equation by factorizing the entire relativistic wave operator into a product of two operators – one of these is the operator for the entire Dirac equation.
The Dirac equation introduces spin matrices, in the particular case of the Dirac equation they are the gamma matrices for spin1/2 particles, and the solutions are 4component spinor fields with two components corresponding to the particle and the other two for the antiparticle. In general, the Hamiltonian to be substituted in the general Schrödinger equation is not just a function of position and momentum operators, but also of spin matrices. Also, the solutions to a relativistic wave equation, for a massive particle of spin s, are complexvalued 2(2s + 1)component spinor fields.
Quantum field theory
The general equation is also valid and used in quantum field theory, both in relativistic and nonrelativistic situations. However, the solution ψ is no longer interpreted as a "wave", but more like a "field".
See also
Notes
References
External links

 Quantum Physics — textbook with a treatment of the timeindependent Schrödinger equation
 Linear Schrödinger Equation at EqWorld: The World of Mathematical Equations.
 Nonlinear Schrödinger Equation at EqWorld: The World of Mathematical Equations.
 directory of the book.
 All about 3D Schrödinger Equation
 Mathematical aspects of Schrödinger equations are discussed on the Dispersive PDE Wiki.
 WebSchrödinger: Interactive solution of the 2D timedependent and stationary Schrödinger equation
 An alternate derivation of the Schrödinger Equation
 Online softwarePeriodic Potential Lab Solves the timeindependent Schrödinger equation for arbitrary periodic potentials.
 What Do You Do With a Wavefunction?
 The Young DoubleSlit Experiment
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