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

A modification of G3 theory incorporating three changes is presented. The t hree 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. Exte nsion of G3 theory in this manner, referred to as G3X, is found to give sig nificantly better agreement with experiment for the G3/99 test set of 376 r eaction energies. Overall the mean absolute deviation from experiment decre ases from 1.07 kcal/mol (G3) to 0.95 kcal/mol (G3X). The largest improvemen t occurs for nonhydrogens. In this subset of energies the mean absolute dev iation 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-Fo ck basis set. In addition, five other G3 methods are modified to incorporat e these new features. Two of these are based on reduced orders of perturbat ion theory, G3X(MP3) and G3X(MP2), and have mean absolute deviations for th e G3/99 test set of 1.13 and 1.19 kcal/mol, respectively. The other three m ethods 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, res pectively. (C) 2001 American Institute of Physics.