Decomposition of the total energy at the hf-scf level and at several levels of correlation. i. a study of the interaction in clusters of he, ne and ar atoms

The interaction energies between two He atoms, two Ne atoms and two Ar atoms were calculated first at the HF-SCF level by a new decomposition scheme using separated molecular orbitals (SMOs). Special attention was paid to the effect of the counterpoise (CP) method. The results show that the CP procedure is an acceptable device for estimating some basis set deficiencies in the systems studied. When using the SMO-LMBPT scheme, on the other hand, the CP calculations become unnecessary. The correlation energy in some He n and Ne n (n = 2, 3, 4) clusters have been studied using the SMO-LMBPT formalism at the second, third and fourth levels of MBPT. The effect of the CP method on the intracorrelated parts was studied and the conclusions are similar to those obtained at the HF-SCF level. Both the intra- and intercorrelated parts, as expected, show transferability properties in the systems studied.