Extension of gaussian-3 theory to molecules containing third-row atoms K, Ca, Ga-Kr

Gaussian-3 (G3) theory is extended to molecules containing the third-row no ntransition elements K, Ca, Ga-Kr. Basis sets compatible with those used in G3 theory for molecules containing first- and second-row atoms have been d erived. The G3 average absolute deviation from experiment for a set of 47 t est reactions containing these elements is 0.94 kcal/mol. This is a substan tial improvement over Gaussian-2 theory, which has an average absolute devi ation of 1.43 kcal/mol for the same set. Variations of G3 theory are also p resented that are based on reduced orders of perturbation theory. These var iations also show similar improvement over the corresponding G2 methods. Th e use of scaling parameters in G3 theory for the third row was investigated and found to perform nearly as well as use of the higher level correction. In addition, these methods are evaluated on a set of molecules containing K and Ca for which the experimental data are not accurate enough for them t o be included in the test set. Results for this set indicate that G3 theory performs significantly better than G2 for molecules containing Ca. When th e 47 third-row systems are added to the G3/99 database the complete G3 aver age absolute deviation becomes 1.06 kcal/mol for 423 energies. (C) 2001 Ame rican Institute of Physics.