GAUSSIAN-3 (G3) THEORY FOR MOLECULES CONTAINING FIRST-ROW AND 2ND-ROWATOMS

Gaussian-3 theory (G3 theory) for the calculation of molecular energie s of compounds containing first (Li-F) and second row (Na-Cl) atoms is presented. This new theoretical procedure, which is based on ab initi o molecular-orbital theory, modifies G2 theory [J. Chem. Phys. 94, 722 1 (1991)] in several ways including a new sequence of single point ene rgy calculations using different basis sets, a new formulation of the higher level correction, a spin-orbit correction for atoms, and a corr ection for core correlation. G3 theory is assessed using 299 energies from the G2/97 test set including enthalpies of formation, ionization potentials, electron affinities, and proton affinities. This new proce dure corrects many of the deficiencies of G2 theory. There is a large improvement for nonhydrogen systems such as SiF4 and CF4, substituted hydrocarbons, and unsaturated cyclic species. Core-related correlation is found to be a significant factor, especially for species with unsa turated rings. The average absolute deviation from experiment for the 148 calculated enthalpies of formation is reduced to under one kcal/mo l, from 1.56 kcal/mol for G2 theory to 0.94 kcal/mol for G3 theory. Si gnificant improvement is also found for ionization potentials and elec tron affinities. The overall average absolute deviation of G3 theory f rom experiment for the 299 energies is 1.02 kcal/mol compared to 1.48 kcal/mol for G2 theory. (C) 1998 American Institute of Physics.