APPLICATION OF GROUP R4 TO ENERGY-LEVEL SEPARATIONS AND EVALUATION OF CORRELATION ENERGY IN ATOMS

The group R4 is applied to the study of energy-level separations and correlation energies in the ground-state configurations of first-row atoms. The separation between two nearly degenerate levels, e.g., 2s2 S1 and 2p2 S1, considering the i>j1rij part of the Hamiltonian only, can then be determined by means of group theory, eliminating the need of solving the secular equation arising from the usual configuration-interaction procedure. Extending this work to the one-electron part of the Hamiltonian enables us to estimate the actual splittings and the ε(2s2) nondynamical (near degeneracy) correlation energy for the first-row atoms. The Z and N dependence of this type of correlation is seen to be closely related to the group theoretical treatment. Finally this method is extended to a treatment of the nondynamical correlation in second-row atoms. © 1969 The American Physical Society.