Second-order correlation energy for H2O using explicitly correlated Gaussian geminals

Abstract

The second-order pair energies are calculated for the H2O molecule employing explicitly correlated Gaussian-type geminals. The selection of the basis functions is guided by the completeness criteria for Gaussian geminal bases of C2v symmetry and by the recently developed technique [J. Chem. Phys. 100, 1366 (1994)] of crude optimization of nonlinear geminal parameters. Several simplifications have been made in the matrix elements evaluations that substantially reduce the optimization time without sacrificing numerical accuracy. The present results provide the new best upper bound to the second-order correlation energy for the water molecule. These results are compared with those obtained by Kutzelnigg and co-workers using the basis set containing terms linear in the interelectronic distance.