Gaussian-3 theory using scaled energies

A modification of Guassian-3 (G3) theory using multiplicative scale factors , instead of the additive higher level correction, is presented. In this me thod, referred to as G3S, the correlation energy is scaled by five paramete rs and the Hartree-Fock energy by one parameter. The six parameters are fit ted to the G2/97 test set of 299 energies and the resulting mean absolute d eviation from experiment is 0.99 kcal/mol compared to 1.01 kcal/mol for G3 theory. The G3S method has the advantage compared to G3 theory in that it c an be used for studying potential energy surfaces where the products and re actants have a different number of paired electrons. In addition, versions of the computationally less intensive G3(MP3) and G3(MP2) methods that use scaled energies are also presented. These methods, referred to as G3S(MP3) and G3S(MP2), have mean absolute deviations of 1.16 and 1.35 kcal/mol, resp ectively. (C) 2000 American Institute of Physics. [S0021-9606(00)30902-3].