Gaussian-3X (G3X) theory: Use of improved geometries, zero-point energies, and Hartree–Fock basis sets

Abstract

A modification of G3 theory incorporating three changes is presented. The three new features include: (1) B3LYP/6-31G(2df,p) geometry; (2) B3LYP/6-31G(2df,p) zero-point energy; and (3) addition of a g polarization function to the G3Large basis set for second-row atoms at the Hartree–Fock level. Extension of G3 theory in this manner, referred to as G3X, is found to give significantly better agreement with experiment for the G3/99 test set of 376 reaction energies. Overall the mean absolute deviation from experiment decreases from 1.07 kcal/mol (G3) to 0.95 kcal/mol (G3X). The largest improvement occurs for nonhydrogens. In this subset of energies the mean absolute deviation from experiment decreases from 2.11 to 1.49 kcal/mol. The increased accuracy is due to both the use of new geometries and the larger Hartree–Fock basis set. In addition, five other G3 methods are modified to incorporate these new features. Two of these are based on reduced orders of perturbation theory, G3X(MP3) and G3X(MP2), and have mean absolute deviations for the G3/99 test set of 1.13 and 1.19 kcal/mol, respectively. The other three methods are based on scaling the energy terms, G3SX, G3SX(MP3), and G3SX(MP2). They have mean absolute deviations of 0.95, 1.04, and 1.34 kcal/mol, respectively.