The effect of disconnected wavefunction clusters of double excitations on the correlation energy of molecules

Abstract

The effect of quadruply excited configurations through the fourth order of MB RSPT on the correlation energy of molecules is studied. The correlation energy of the BH. H 2O and N 2 molecules is calculated in such a way that the second plus third order of MB RSPT are included fully, and from the fourth order of MB RSPT only those diagrams are included which correspond to quadruply excited configurations. This correlation energy is compared with that obtained by CI calculations including quadruply excited configurations. Two effects can be studied by these calculations, namely, the size inconsistency terms in CI, as well as the net quadruple excitation effect, i.e. the further lowering of the correlation energy by quadruply excited configurations.