Gaussian-3 theory: a variation based on third-order perturbation theory and an assessment of the contribution of core-related correlation

A variation of Gaussian-3 (G3) theory is presented in which third-order perturbation theory is used in place of fourth-order perturbation theory to decrease the computational requirements. This method, referred to as G3(MP3) theory, is assessed on 299 energies from the G2/97 test set. The average absolute deviation from experiment of G3(MP3) theory for the 299 energies is 1.22 kcal/mol. The importance of core-related correlation in various G3 methods is also discussed.