G3X¹ is a modification of G3 theory² incorporating three changes. The three new features include (1) B3LYP/6-31G(2df,p) geometry, (2) B3LYP/6-31G(2df,p) zero-point energy, and (3) addition of a g polarization function to the G3Large basis set for second-row atoms at the Hartree-Fock level. G3X is found to give significantly better agreement with experiment for the G3/99 test set of 376 reaction energies. Overall the mean absolute deviation from experiment decreases from 1.07 kcal/mol (G3) to 0.95 kcal/mol (G3X). In addition, five other G3 methods have been modified to incorporate these new features. Two of these are based on reduced orders of perturbation theory, G3X(MP3) and G3X(MP2), and have mean absolute deviations for the G3/99 test set of 1.13 and 1.19 kcal/mol, respectively. The other three methods are based on scaling the energy terms, G3SX, G3SX(MP3), and G3SX(MP2). They have mean absolute deviations of 0.95, 1.04, and1.34 kcal/mol, respectively.

• Energies for the G3/99 Test Set: Neutrals, Ions, Atomic Species
• G3Xlarge basis set
• Bibliography

1. "Gaussian-3x (G3X) Theory: Use of Improved Geometries, Zero-Point Energies, and Hartree-Fock Basis Sets," L. A. Curtiss, P. C. Redfern, K. Raghavachari, and J. A. Pople, J. Chem. Phys., 114, 108-117. (2001).

5. "Assessment of Gaussian-3 and Density Functional Theories for a Larger Test Set" L. A. Curtiss, K. Raghavachari, P. C. Redfern, and J. A. Pople, Journal of Chemical Physics, 112 7374 (2000).

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