SEARCH Articles Figures Tables Atomic theory Atomic-Partitioned Symmetry-Adapted Perturbation Theory Atoms degenerate perturbation theory Atoms theory Density functional perturbation theory atomic charge Excited States of the Helium Atom. Degenerate Perturbation Theory Helium atom energy from perturbation theory Perturbation theory applied to helium atom Perturbation theory applied to hydrogen atom Perturbation theory applied to hydrogen atom in electric field Perturbation theory for the energy of an atom