= Nondegenerate perturbation theory
<Perturbation theory> for an isolated <eigenvalue> with a one-dimensional <eigenspace>. <First-order nondegenerate perturbation theory> gives the diagonal perturbation expectation, while <second-order nondegenerate perturbation theory> accounts for coupling to other unperturbed eigenstates through energy denominators. Symmetry can permit its use within one block even if another, uncoupled symmetry block has the same unperturbed eigenvalue.
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