A complete basis set model chemistry. IV. An improved atomic pair natural orbital method

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Abstract
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., D0 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.