AN EXPLICITLY CORRELATED COUPLED-CLUSTER CALCULATION OF THE HELIUM-HELIUM INTERATOMIC POTENTIAL

Explicitly correlated coupled cluster (CCSDT-1a-R12) results were obtained for the He-2 interatomic potential from a new, integral-direct implementation. With the new code, Gaussian basis sets as large as 11s8p6d5f4g3h could be employed, and the potential energy curve was calculated over a wide range using a basis of the type 11s8p6d5f4g. This curve is very close to represent the basis set limit of the CCSDT-1a approach. At the internuclear separation R=5.6 a(0), the CCSDT-1a limiting value for the interaction energy is -10.68 K. As the effect of quadruple substitutions can be estimated as -0.32 K, this limiting value is perfectly consistent with the accurate quantum Monte Carlo calculation of Anderson et al. [J. Chem. Phys. 99, 345 (1993)], who reported a well depth of -11.01+/-0.10 K. On the other hand, however, CCSDT-1a-R12 calculations of the He-2 potential energy curve strongly indicate that the most recent semiempirical potentials available in the literature are slightly too repulsive for short (R less than or equal to 4.0 a(0)) interatomic distances. (C) 1995 American Institute of Physics.