BENCHMARK CALCULATIONS WITH CORRELATED MOLECULAR WAVE-FUNCTIONS .5. THE DETERMINATION OF ACCURATE AB-INITIO INTERMOLECULAR POTENTIALS FOR HE-2, NE-2, AND AR-2

Dimer interactions of helium, neon, and argon have been studied using the augmented correlation consistent basis sets of Dunning and co-workers. Two correlation methods have been employed throughout; Moller-Plesset perturbation theory through fourth-order (MP4) and single and double excitation coupled-cluster theory with perturbative treatment of triple excitations [CCSD(T)]. Full configuration interaction (FCI) calculations were performed on He-2 for some basis sets. In general, only valence electrons were correlated, although some calculations which also correlated the n = 2 shell of Ar-2, were performed. Dimer potential energy curves were determined using the supermolecule method with and without the counterpoise correction. A series of additional basis sets beyond the augmented correlation consistent Sets were explored in which the diffuse region of the radial function space has been systematically saturated. In combination with the systematic expansion across angular function space which is inherent to the correlation consistent prescription, this approach guarantees very accurate atomic polarizabilities and hyperpolarizabilities and should lead to an accurate description of dispersion forces. The best counterpoise-corrected MP4 values for the well depths of the three dimers are (in microhartrees, empirical values in parentheses) He-2, 31.9 (34.6); Net, 123 (134); and Ar-2, 430 (454). The corresponding CCSD(T) values are He-2, 33.1; Ne-2, 128; and Ar-2, 417. Although these values are very good, the nearly exponential convergence of well depth as a function of basis quality afforded by using the various series of correlation consistent basis sets allows estimation of complete basis set (CBS) limiting values. The MP4 estimated CBS limits are He-2, 32.2; Ne-2, 126; and Ar-2, 447. The corresponding CCSD(T) values are He-2, 33.4; Ne-2, 130; and Ar-2, 430. Equilibrium separations are also reproduced with similar accuracy.