Many-Body Perturbation-Theory Calculations for Excited Molecular States

The folded-diagram procedure of Brandow, suitable for many-body perturbation-theory calculations when unperturbed functions are degenerate, is applied to the four lowest excited ^+ states of H₂ at R=1. 4 bohr. Calculating energies up to third order (inclusive) and estimating higher-order corrections, an agreement of 1 kcal/mole is obtained with the accurate results of Kolos and Wolniewicz.