BASIS-SET SUPERPOSITION PROBLEM IN INTERACTION ENERGY CALCULATIONS WITH EXPLICITLY CORRELATED BASES - SATURATED 2ND-ORDER AND 3RD-ORDER ENERGIES FOR HE-2

Explicitly correlated basis set of Gaussian-type geminals has been emp loyed in supermolecular calculations of the interaction energy of two helium atoms using the second- and third-order of the many-body pertur bation theory and the Moller-Plesset partitioning of the Hamiltonian. A geminal extension of the counterpoise procedure of Boys and Bernardi has been proposed to correct for the basis set superposition error. P erformance of the proposed correction scheme has been analyzed at the second-order level using a sequence of geminal bases varying in the de gree of completeness in representing the intra- and intermonomer corre lation effects. The nonlinear parameters of these bases were optimized by minimizing the second-order energy of the helium atom and the seco nd-order dispersion energy of the He dimer. The best upper bounds to d ate have been obtained for both quantities. The numerical results show that the counterpoise procedure should be used at all levels of basis set completeness. By employing the union of the largest of the obtain ed bases and reoptimizing some of the nonlinear parameters using the c omplete second-order energy functional for the dimer, the best estimat es to date of the second- and third-order supermolecular interaction e nergies for He-2 have been computed. At the minimum interatomic separa tion these energies are estimated to be accurate to 0.01 K or better. Adding higher-order terms computed using orbital bases, leads to a hel ium dimer interaction potential with the depth of 11.00 K, somewhat la rger than current experimental results. (C) 1996 American Institute of Physics.