Singlet-triplet differences in the lowest Πustates of H₂

A study of the energy differences between the 1 Pi u and 3 Pi u states of H 2 has been undertaken, and differences in kinetic energies, electron-nuclear attraction energies, and interelectronic repulsion energies are also given for a range of internuclear separations 0.5 au<or=R<or=5.0 au. Wavefunctions for both states were calculated as part of the study, using a basis set of twenty Slater-type orbitals to do a self-consistent-field calculation followed by configuration interaction. Optimized exponents were used for the most important orbitals at each internuclear separation. It has been found that the interelectronic repulsion energy of the triplet is greater than that of the singlet, and that the triplet lies lower in energy because of its greater electron-nuclear attraction energy.