Abstract
Energy differences, and differences in kinetic energies, nuclear attraction energies and interelectronic repulsion energies are given for the lowest 1,3 Pi states of H2, HeH+, and LiH++, and an analysis of the results in terms of 1/Z-expansions is presented. If Z is used to denote the charge of the nucleus of higher atomic number, then it is found that at low Z the difference in interelectronic repulsion is dominated by higher-order terms, and that the often-quoted statement that the singlet-triplet energy difference is twice an exchange integral is true only for high values of the nuclear charge Z.