Degenerate stationary perturbation
Inside a degenerate eigenspace of H₀, first-order energy shifts and appropriate zeroth-order states come from diagonalizing the projected Hermitian perturbation PVP.
Key knowledge
Inside a degenerate eigenspace of H₀, first-order energy shifts and appropriate zeroth-order states come from diagonalizing the projected Hermitian perturbation PVP. Coupling to other energy subspaces enters higher orders.
Values and equations
(PVP)|χ_a⟩=δE_a^(1)|χ_a⟩; E_a≈E₀+δE_a^(1)
- P
- Subspace projector
- V,E₀,δE_a^(1),E_a
- Perturbation and energies J
- χ_a
- Normalized state in the degenerate subspace
Conditions and scope
- P projects onto the specified finite degenerate subspace, with controlled coupling to separated external levels.
- Zero eigenvalues or a singular PVP matrix do not prevent Hermitian diagonalization; further unresolved degeneracy can need higher order.
Sources · 1
References
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Evidence and review
Quantum Mechanics (2016): Degenerate perturbation theory: Equations 7.71–7.78: projected matrix eigenproblem; rejects source's unnecessary nonzero-determinant restriction.
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Entry details
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