Gaussian-3X (G3X) theory using coupled cluster and Brueckner energies

Variations of Gaussian-3X (G3X) theory are presented having the quadratic configuration interaction (QCISD(T)) energy calculation replaced by either a coupled cluster (CCSD(T)) or Brueckner (BD(T)) energy. This modification is reported for several G3X methods including G3X, G3X(MP3), G3X(MP2), G3SX, and G3SX(MP3). In most cases the replacement of the QCISD(T) energy by one of these alternative energies results in a slight improvement in the accuracy of these methods as assessed on the G3/99 test set of molecules. These new G3X methods are useful alternatives to the G3X methods based on quadratic configuration interaction.