A COMPLETE BASIS-SET MODEL CHEMISTRY .4. AN IMPROVED ATOMIC PAIR NATURAL ORBITAL METHOD

An improved complete basis set-quadratic configuration interaction/atomic pair natural orbital (CBS-QCI/APNO) model is described in this paper. It provides chemical energy differences (i.e., D-0 I.P., and E.A.) with a mean absolute error of 0.53 kcal/mol for the 64 first-row examples from the G2 test set, and is computationally feasible for species with up to three first-row atoms. A set of 20 CBS-QCI/APNO bond dissociation energies of hydrocarbons also agree with known experimental values to within less than 1 kcal/mol. Calculations on the cyclopropenyl radical and cyclopropenylidene provide new dissociation energies which are in accord with an interpretation of the thermochemistry emphasizing ring strain and aromaticity.