Ab initio calculations of intermolecular potentials

Abstract

The Ar₂ ground state potential curve is calculated using a triple zeta + 2d basis set. The so-called superposition error is easily estimated at the SCF level. A subsequent second order Moller-Plesset perturbative CI using delocalized SCF MOs (which does not distinguish between intra- and interatomic correlation corrections) somewhat overestimates the depth of the potential well. Turning back to localized SCF MOs one may reach satisfactory results by adding the dispersion interatomic double excitation corrections, as previously done for He₂, but the change in the intra-atomic correlation and the superposition error at the CI level could not be estimated. A new procedure is derived, which does not neglect the variation of the intraatomic correlation energy and estimates the superposition error at both the SCF and the CI level. It consists of using the same molecular set of virtual MOs for the molecule and for the atoms, and closely related occupied MOs, namely the SCF localized MOs for the molecule and the projection of the atomic SCF orbitals into the molecular Fock space for the atoms. This choice allows a term to term comparison of the contributions and the appropriate cancellations, leading to satisfactory results.