Extrapolation to the complete basis set limit without counterpoise. The pair potential of helium revisited.

Calculations using the full configuration interaction and coupled cluster methods are combined to yield a realistic curve for the helium dimer by extrapolation to the complete basis set limit. Extrapolations of the full configuration interaction energies alone are also done by reassignment of the basis hierarchic numbers. In no case has the common procedure of correcting the energies for the basis set superposition error by counterpoise been employed. The curves so generated agree well with the best available estimates, in one case within 1 cK (centikelvin) at the minimum. This corroborates our previous finding that such an approach provides an alternative to counterpoise that is more economical and general, besides yielding a more accurate curve. It may also conceivably justify that extrapolation to the complete basis set limit is, per se, all that is necessary to get a reliable potential function.