SEARCH Articles Figures Tables Atoms degenerate perturbation theory Degenerate perturbation theory effective Hamiltonian calculations Degenerate-Level Perturbation Theory by Inspection Degenerate-level perturbation theory Electronic states degenerate perturbation theory Excited States of the Helium Atom. Degenerate Perturbation Theory First-order Perturbation Theory for a Non-degenerate Level Mixing of Degenerate Orbitals— First-Order Perturbations Mixing of Non-Degenerate Orbitals— Second-Order Perturbations Multi-configuration quasi-degenerate perturbation theory Multiconfiguration quasi-degenerate perturbation theory Non-degenerate perturbation Perturbation Theory for a Degenerate Energy Level Perturbation Theory for a Degenerate State Perturbation degenerate orbitals Perturbation methods degenerate case Perturbation theories degenerate problems Perturbation theory degenerate Perturbation theory degenerate states Perturbation theory degenerate systems Perturbation theory non-degenerate Quasi-degenerate perturbation theory Quasi-degenerate perturbation theory MC-QDPT) Quasi-degenerate perturbation theory QDPT) Time-Independent Degenerate Perturbation Theory