Correlation energy of doublet and triplet electronic states is formulated in terms of many-body diagrammatic perturbation theory through third order. Applications are presented for B${\mathrm{H}}_2$ and N${\mathrm{H}}_2$ radicals and the computed correlation energies are compared with data obtained with the same basis set from configuration-interaction treatments covering singly and doubly excited configurations. It is shown that the two approaches give correlation energies close in absolute value.

• Received 2 October 1979

DOI:https://doi.org/10.1103/PhysRevA.22.2392