Perturbative mixing parameter
= Perturbative mixing parameter
The ratio $|\langle k|V|i\rangle/(E_i-E_k)|$ measures a leading admixture between unperturbed eigenstates. Small <matrix elements> compared with relevant gaps justify a nondegenerate expansion locally. A small overall perturbation does not suffice near a coupled degeneracy, where the nearly degenerate block needs diagonalization.