quasidegenerate
Quasidegenerate, often written quasi-degenerate, refers to a situation in which two or more eigenvalues of a system’s Hamiltonian are very close in value but not exactly equal. The concept is common in quantum mechanics, chemistry, and related fields, where energy levels, electronic configurations, or vibrational modes nearly coincide. It contrasts with exact degeneracy, where energies are identical due to symmetry or other constraints.
In perturbation theory, quasidegeneracy signals the breakdown of nondegenerate approaches because small perturbations can cause substantial
Applications are widespread. In chemistry, quasidegenerate electronic states occur when frontier orbitals are close in energy,
Numerical aspects include challenges in solving near-degenerate eigenproblems, where standard methods may be ill-conditioned. Careful treatment