Gaussian-1 theory of molecular energies for second-row compounds

The Gaussian-1 theoretical procedure is extended and tested on compounds containing second-row atoms (Na–Cl). This is a composite procedure based on ab initio molecular orbital theory, utilizing large basis sets (including diffuse-sp, double-d, and f-polarization functions) and treating electron correlation by Mo/ller–Plesset perturbation theory and by quadratic configuration interaction. Total atomization energies for a set of 24 species agree with accurate experimental data to an accuracy of better than 3 kcal/mol in most cases, SO2 being the notable exception. Similar agreement is achieved for ionization energies, electron affinities, and proton affinities. The method is used to assess experimental data for a number of other compounds having less accurate atomization energies.