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

Abstract

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 3s3p2d2 f1g midbond functions. The basis was specifically designed to optimally describe the [self-consistent field+second-order Möller–Plesset (SCF+MP2)] He2 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 He2 well depth is found to be ΔE(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 Möller–Plesset polarization dispersion energy Edisp(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.