Gaussian-3 Theory using Coupled Cluster Energies

Previously, we have described a general theoretical procedure referred to as Gaussian-3 (G3) theory for the quantum chemical computation of total energies of molecules at their equilibrium geometries. G3 theory is based on the quadratic configuration interaction and Moller-Plesset perturbation techniques for the evaluation of accurate electron correlation energies. However, the quadratic configuration interaction method is not available in the quantum chemistry codes used by some research groups. In addition, there are occasional cases where the quadratic configuration interaction method fails to yield accurate energies. In this work, we present variations of G3 theory in which the quadratic configuration interaction energy calculation is replaced by a coupled cluster energy calculation. The new methods are assesed on enthalpies of formation, ionization potentials, electron affinities, and proton affinities of a test set of molecules where accurate experimental information is available. The new G3 methods based on coupled cluster theory perform with an accuracy comparable to that of the original G3 theory, and are useful alternatives in cases where quadratic configuration interaction may be deficient.