Non-degenerate perturbation theory

Definition

Non-degenerate perturbation theory is a method in quantum mechanics used to find an approximate solution to a quantum system when a small perturbation is applied, assuming that the energy levels of the unperturbed system are distinct. This technique is useful for calculating changes in the energy levels and eigenstates of a system when subjected to small external influences, without the complications that arise from degenerate states where multiple states share the same energy level.

5 Must Know Facts For Your Next Test

1. In non-degenerate perturbation theory, only first-order corrections to energy and wave functions are typically calculated unless higher precision is required.
2. The first-order energy correction for a state |n⟩ due to a perturbation V is given by the expectation value ⟨n|V|n⟩.
3. The first-order correction to the wave function involves contributions from all other states, which makes it dependent on their respective overlap integrals with the perturbed state.
4. Non-degenerate perturbation theory assumes that no two energy levels are the same, simplifying calculations compared to degenerate cases where levels can mix.
5. This theory can be extended to higher orders, but its validity diminishes if the perturbation is not sufficiently small or if degeneracies appear in the system.

Review Questions

• How does non-degenerate perturbation theory simplify calculations compared to cases involving degenerate states?
  • Non-degenerate perturbation theory simplifies calculations by focusing on systems where each energy level is unique, eliminating the complexity of state mixing that occurs in degenerate cases. When energy levels are degenerate, multiple eigenstates correspond to the same energy, leading to potential interactions that complicate both energy corrections and wave function adjustments. By assuming distinct energy levels, this approach allows for straightforward application of perturbative techniques without having to account for these additional interactions.
• Discuss how the first-order correction in non-degenerate perturbation theory impacts both energy levels and wave functions.
  • In non-degenerate perturbation theory, the first-order correction for energy levels is directly calculated using the expectation value of the perturbing potential with respect to the unperturbed state. For wave functions, however, first-order corrections involve contributions from all other states in the system, weighted by their overlaps with the perturbed state. This dual effect allows us to understand how external influences shift not only energy levels but also alter the nature of quantum states themselves in a clear manner.
• Evaluate the limitations of non-degenerate perturbation theory when dealing with larger perturbations or near-degenerate states.
  • Non-degenerate perturbation theory becomes less reliable when dealing with larger perturbations since it relies on the assumption that changes to energy levels and states are small. If the perturbation significantly alters the system's properties, higher-order corrections may become necessary for accuracy, complicating computations. Additionally, near-degenerate states present challenges as they can mix under perturbations, violating the fundamental assumptions of non-degeneracy and requiring different techniques, such as degenerate perturbation theory, to properly analyze their behavior.

Related terms

Perturbation Theory: A mathematical approach used to find an approximate solution to a problem by starting with a known solution of a simpler problem and adding corrections due to small disturbances.

Eigenstates: States of a quantum system that correspond to definite values of observable quantities, such as energy, and are solutions to the time-independent Schrödinger equation.

Variational Principle: A principle stating that the best approximation to the ground state energy of a quantum system can be found by minimizing the expectation value of the Hamiltonian with respect to trial wave functions.