NEW VERSIONS OF APPROXIMATELY EXTENSIVE CORRECTED MULTIREFERENCE CONFIGURATION-INTERACTION METHODS

In a previous paper (Szalay, P. G.; Bartlett, R. J. J. Chem. Phys. 199 5, 103, 3600) we reviewed all approximately extensive corrected multir eference configuration interaction (MR-CI) methods and also proposed a new method called the multireference averaged quadratic coupled-clust er (MR-AQCC) method. In this paper we continue the investigation of th ese methods and propose new variants by including the effect of (a) vi rtual orbitals in the EPV (exclusion principle violating) contribution s, (b) redundancies due to the MR nature of the reference space. Resul ts of test calculations on water, Be-2, Be/H-2, ozone and NO2/HF syste ms are presented which show that these new methods are able to describ e potential energy surfaces very accurately even if small, but qualita tively correct, reference spaces are used.