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

Explicitly correlated coupled cluster (CCSDT-1a-R12) results were obta ined for the He-2 interatomic potential from a new, integral-direct im plementation. With the new code, Gaussian basis sets as large as 11s8p 6d5f4g3h could be employed, and the potential energy curve was calcula ted over a wide range using a basis of the type 11s8p6d5f4g. This curv e is very close to represent the basis set limit of the CCSDT-1a appro ach. 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 quadrup le substitutions can be estimated as -0.32 K, this limiting value is p erfectly consistent with the accurate quantum Monte Carlo calculation of Anderson et al. [J. Chem. Phys. 99, 345 (1993)], who reported a wel l depth of -11.01+/-0.10 K. On the other hand, however, CCSDT-1a-R12 c alculations of the He-2 potential energy curve strongly indicate that the most recent semiempirical potentials available in the literature a re slightly too repulsive for short (R less than or equal to 4.0 a(0)) interatomic distances. (C) 1995 American Institute of Physics.