Argon pair potential at basis set and excitation limits

A new ab initio interaction potential for the electronic ground state of argon dimer has been developed. The potential is a sum of contributions corresponding to various levels of the coupled-cluster theory up to the full coupled-cluster method with single, double, triple, and quadruple excitations. All contributions have been calculated in larger basis sets than used in the development of previous Ar-2 potentials, including basis sets optimized by us up to the septuple(sextuple)-zeta level for the frozen-core (all-electron) energy. The diffuse augmentation functions have also been optimized. The effects of the frozen-core approximation and the relativistic effects have been computed at the CCSD(T) level. We show that some basis sets used in literature to compute these corrections may give qualitatively wrong results. Our calculations also show that the effects of high excitations do not necessarily converge significantly faster (in absolute values) in basis set size than the effects of lower excitations, as often assumed in literature. Extrapolations to the complete basis set limits have been used for most terms. Careful examination of the basis set convergence patterns enabled us to determine uncertainties of the ab initio potential. The interaction energy at the near-minimum interatomic distance of 3.75 angstrom amounts to -99.291 +/- 0.32 cm(-1). The ab initio energies were fitted to an analytic potential which predicts a minimum at 3.762 angstrom with a depth of 99.351 cm(-1). Comparisons with literature potentials indicate that the present one is the most accurate representation of the argon-argon interaction to date. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3478513]