Validity of additivity approximations used in GAUSSIAN-2 theory

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Abstract
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. 9 4, 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.