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Three additivity approximations used in GAUSSIAN-2 (G2) theory to compute effective QCISD(T)/6-311+G(3df,2p) energies for molecular systems are investigated by carrying out full QCISD(T)/6-311+G(3df,2p) calculations on the set of 125 systems used for validation J. Chem. Phys. 94, 7221 (1991). The results indicate that the approximations work very well. The average absolute deviation of the full results from experiment is 1.17 kcal/mol which is close to that of G2 theory (1.21 kcal/mol). The average absolute deviation with G2 theory is only 0.30 kcal/mol. However, the cpu cost increases by a factor of 2–3.
Curtiss et al. (Mon,) studied this question.