Pair-correlation energies in sodium hydride with many-body perturbation theory

Many-body perturbation theory (MBPT) is applied to the electronic structure of the sodium hydride molecule using a basis set of Slater-type orbitals. Both a V^N and a V^N-1 potential are considered for determining the virtual orbitals, but these are shown to give nearly equivalent results when all pairlike diagrams are summed through third order in the MBPT expansion using shifted denominators. A value of -0. 2544 hartrees is obtained for the correlation energy, which is 59% of the experimental value. A clear separation into intrashell and intershell correlation energy effects is observed.