Extension of Gaussian-2 (G2) theory to molecules containing third-row atoms K and Ca

Gaussian-2 (G2) theory has been extended to molecules containing the third-row nontransition elements K and Ca. Basis sets compatible with those used in G2 theory for molecules containing first- and second-row atoms, as well as the third-row elements Ga–Kr, have been derived. As for Ga–Kr, first-order spin–orbit corrections are included explicitly in the G2 energies of K- and Ca-containing atoms and molecules. In contrast to the procedure for Ga–Kr, the 3s and 3p orbitals need to be included in the correlation space for K- and Ca-containing molecules. The geometries obtained with the new basis sets are in satisfactory agreement with experiment. Assessment of the thermochemical predictions is limited because of the sparsity of accurate experimental data for molecules containing K and Ca. Ionization energies are generally in good agreement with experimental values, as are the atomization energies for K-containing molecules. However, there are larger differences between theoretical and experimental atomization energies for Ca-containing molecules.