G3S theory¹ is a modification of G3 theory² using scaled energies, instead of the higher level correction. In it the correlation energy is scaled by four parameters and the Hartree-Fock energy by one parameter. The five parameters are fitted to the G2/97 test set^3,4 of 299 energies and the resulting mean absolute deviation from experiment is 1.02 kcal/mol compared to 1.01 kcal/mol for G3 theory. The G3S methods has the advantage compared to G3 theory in that it can be used for studying potential energy surfaces where the products and reactants have a different number of paired electrons. In addition, versions of the cheaper G3(MP2)⁵ and G3(MP3)⁶ methods that use scaled energies are presented. These methods, referred to as G3S(MP3) and G3S(MP2), have absolute mean deviations of 1.16 and 1.36 kcal/mol, respectively. The basis sets used in these methods are the same as in the corresponding G3 methods based on the higher level correction.

• Energies for G2/97 Test Set
• Neutrals
• Ions
• Atomic Species
• Bibliography
• G2/97 Test Set

1. Gaussian-3 theory using Scaled Energies, L. A. Curtiss, K. Raghavachari, P. C. Redfern, and J. A. Pople, Journal of Chemical Physics, 112, 1125 (2000).

2. "Gaussian-3 theory for molecular energies of first- and second-row compound" L.A. Curtiss, K. Raghavachari, P.C. Redfern, V.Rassolov, and J. A. Pople, Journal of Chemical Physics, 109, 7764 (1998).

3. "Assessment of Gaussian-2 and Density Functional Methods for the Computation of Enthalpies of Formation" L. A. Curtiss, K. Raghavachari, P. C. Redfern, and J. A. Pople, Journal of Chemical Physics 106, 1063 (1997).

4. "Assessment of Gaussian-2 and Density Functional Methods for the Computation of Ionization Energies and Electron Affinities" L. A. Curtiss, P. C. Redfern, K. Raghavachari, and J. A. Pople, Journal of Chemical Physics 109, 42 (1998).

5. "Gaussian-3 Theory Using Reduced Moller-Plesset Orders," L. A. Curtiss, P. C. Redfern, K. Raghavachari, V. Rassolov, and J. A. Pople, Journal of Chemical Physics, , 110, 4703 (1999).

6. Gaussian-3 Theory: A Variation Based on Third-Order Perturbation Theory and an Assessment the Contribution of Core-Related Correlation, L. A. Curtiss, P. C. Redfern, K. Raghavachari, and J. A. Pople, Chemical Physics Letters, 313, 600-607 (1999).

For more information contact Larry Curtiss, e-mail:  curtiss@anl.gov