Gaussian-3 Theory: A Variation Based on Third-Order Perturbation Theory and an Assessment of the Contribution of Core-related Correlation

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. The G3 theory has been assessed on a total of 299 energies (enthalpies of formation, ionization energies, electron affinities, and proton affinities) where accurate experimental information is available. The average absolute deviation from experiment of G3 theory for these energies is 1.01 kcal/mol. Though highly accurate, some of the correlation methods used in G3 theory are computationally intensive and it is of interest to find modifications to reduce the computational requirements. In this work, a variation of Gaussian-3 (G3) theory is presented in which the basis set extensions are obtained at the third-order Moller-Plesset perturbation level of theory (in place of the more computationally demanding fourth-order). This method, referred to as G3(MP3) theory, is assessed on the same test set of molecules. The average absolute deviation from experiment of G3(MP3) theory for the 299 energies is 1.22 kcal/mol. The new method provides significant savings in computational time compared to G3 theory and can be applied to larger molecules. The importance of core-related correlation in various G3 methods is also discussed.