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A variation of Gaussian-3 (G3) theory is presented in which the basis set extensions are obtained at the second-order Mo” ller–Plesset level. This method, referred to as G3 (MP2) theory, is assessed on 299 energies from the G2/97 test set J. Chem. Phys. 109, 42 (1998). The average absolute deviation from experiment of G3 (MP2) theory for the 299 energies is 1.30 kcal/mol and for the subset of 148 neutral enthalpies it is 1.18 kcal/mol. This is a significant improvement over the related G2 (MP2) theory J. Chem. Phys. 98, 1293 (1993), which has an average absolute deviation of 1.89 kcal/mol for all 299 energies and 2.03 kcal/mol for the 148 neutral enthalpies. The corresponding average absolute deviations for full G3 theory are 1.01 and 0.94 kcal/mol, respectively. The new method provides significant savings in computational time compared to G3 theory and, also, G2 (MP2) theory.© 1999 American Institute of …