Ab initio computation of molecular singlet-triplet energy differences using auxiliary field Monte Carlo

A method for computing the singlet-triplet energy difference (STED) of mole cules with nearly degenerate states is described. It is based on the shifte d contour auxiliary field Monte Carlo, implemented with plane waves and pse udopotentials. Two additional critical elements are: a variational multiref erence approach, for treating non-dynamical correlation and direct correlat ed sampling of energy differences. Applications to the H-He-H and CH2 syste ms are given, estimating STED of - 14.3 +/-0.1 and 9.8 +/-0.3 kcal/mol, res pectively. The overall accuracy is limited by the type of pseudopotential u sed, with preference to those based on generalized gradients. (C) 2001 Else vier Science BN. All rights reserved.