Nondegenerate stationary perturbation
For an isolated nondegenerate eigenvalue of H₀ and a small Hermitian perturbation V, the leading energy corrections are ⟨n|V|n⟩ and Σ_{k≠n}|⟨k|V|n⟩|²/(E_n⁰−E_k⁰).
Key knowledge
For an isolated nondegenerate eigenvalue of H₀ and a small Hermitian perturbation V, the leading energy corrections are ⟨n|V|n⟩ and Σ_{k≠n}|⟨k|V|n⟩|²/(E_n⁰−E_k⁰). The leading state correction is Σ_{k≠n}⟨k|V|n⟩|k⟩/(E_n⁰−E_k⁰).
Values and equations
E_n≈E_n⁰+V_nn+Σ_{k≠n}|V_kn|²/(E_n⁰−E_k⁰); V_kn=⟨k|V|n⟩
- E_n,E_n⁰,V,V_kn
- Energies, perturbation and matrix elements J
- n,k
- Discrete eigenstate indices
Conditions and scope
- Complete discrete unperturbed orthonormal eigenbasis, nonzero relevant gaps and controlled small coupling-to-gap ratios; sums must converge.
- Near degeneracy requires degenerate or other methods; a formal series need not converge globally.
Sources · 1
References
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Evidence and review
Quantum Mechanics (2016): Non-degenerate perturbation theory: Equations 7.19–7.33: corrections and coupling/gap assumption; the source's energy-zero-dependent diagonal ratio is not required here.
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Entry details
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