Perturbation theory (quantum mechanics)

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In quantum mechanics, perturbation theory is a set of approximation schemes directly related to mathematical perturbation for describing a complicated quantum system in terms of a simpler, known system. The idea is to start with a simple system for which a mathematical solution is known (e.g. the time-independent Schrödinger equation: H ^ | Ψ ⟩ = E | Ψ ⟩ {\displaystyle {\hat {H}}|\Psi \rangle =E|\Psi \rangle } ) and add an additional "perturbing" Hamiltonian ( H ′ {\displaystyle H'} ) representing a weak disturbance to the known system to the original Hamiltonian ( H 0 {\displaystyle H_{0}} )...

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