A VARIATIONAL PERTURBATION THEORY STUDY OF SOME TWO-ELECTRON ATOMIC SYSTEMS, WITH PARTICULAR EMPHASIS ON RELATIVISTIC CORRECTIONS TO THE ENERGY FOR THE STATES (1s)2 singlet S AND (2p)2 triplet P. PART I. THEORY AND RESULTS,

The Hylleraas-Scherr-Knight variation perturbation procedure has been reviewed, and a few extensions and modifications have been suggested. In particular, an upper bound theorem for the energy has been established. The procedure has been applied to the two-electron atomic states 1s2 singlet S and 2p2 triplet P through 20th perturbation order. Perturbation coefficients and expectation values for a number of important operators have been computed. Particular emphasis has been laid upon the evaluation of relativistic and radiative energy corrections for the two states. The results are compared - where possible - with the values obtained by conventional variational calculations, and with experimental values. Author