MRCI calculations on the helium dimer employing an interaction optimized basis set

Multireference configuration interaction (MRCI) calculations are presented for the helium dimer employing a new, 301-orbital basis set consisting of a [8s]6p5d4f3g2h atom-centered set and a set of 3s3p2d2f1g midbond functions. The basis was specifically designed to optimally describe the [self-consistentfield + second-order Moller-Plesset (SCF+MP2)]He(2) interaction energy (at R = 4.0 bohr), using a procedure recently developed for calculations on the water dimer. The MRCI calculations employ an 78-configuration reference set, which is capable of reaching full CI quality. Using the difference between the limiting (coupled-cluster single double triple [CCSD(T)]) values of Klopper and Noga [J. Chem. Phys. 103, 6127 (1995)] and the present values in order to account for basis set incompleteness, the final He(2) well depth is found to be Delta E(R = 5.6) = -10.99 +/- 0.02 K while at R = 4.0 bohr one finds 292.72 +/- 0.2 K. These values are in excellent agreement with the results of Komasa and Rychlewski [Mol. Phys. 91, 909 (1997)], obtained in variational calculations using explicitly correlated Gaussian-type geminal wave functions. This paper also considers the role of the Moller- Plesset polarization dispersion energy E(disp) (20) as well as the correlated first-order interaction energy E((1)), as arising in the symmetry-adapted perturbation theory (SAPT) theory, in determining the binding energy at different levels of correlation [MP2, MP3, CCSD(T) and 78-MRCI]. From these results it appears that the SAPT calculations of Korona et al. [J. Chem. Phys. 106, 5109 (1997)] incorrectly predicted significantly more attraction at both distances. (C) 1999 American Institute of Physics. [S0021-9606(99)30222-1].