Open shell systems are treated using unrestricted wavefunctions (UHF, UMP2 . . )
The higher level correction (HLC) is supposed to compensate for the remaining deficiencies
of the method. In contrast to G2 theory, different parameters A and B are used for
atoms and molecules to give the smallest average absolute deviation from experiment.
Spin orbit correction terms E(SO) (mainly of experimental origin) are added only for atoms
The mean absolute deviation for the original G2 neutral set (125 reaction energies) is
1.00 kcal/mol. The mean absolute deviation for the extended G2 neutral set (148 reaction
energies) is 0.9 kcal/mol.
L. A. Curtiss, K. Raghavachari, P. C. Redfern, V. Rassolov, J. A. Pople,
"Gaussian-3 (G3) theory for molecules containing first and second-row atoms" J. Chem. Phys.1998, 109, 7764 - 7776.
L. A. Curtiss, K. Raghavachari,
"G2 Theory" The Encyclopedia of Overview of Gaussian Theories, P. v. R. Schleyer (editor-in-chief),
John Wiley & Sons Ltd, Athens, USA, 1998, 2, 1104 - 1114.
L. A. Curtiss, P. C. Redfern, K. Raghavachari, V. Rassolov, J. A. Pople,
"Gaussian-3 theory using reduced Møller-Plesset order" J. Chem. Phys.1999, 110, 4703 - 4709.
A. G. Baboul, L. A. Curtiss, P. C. Redfern, K. Raghavachari,
"Gaussian-3 theory using density functional geometries and zero-point energies" J. Chem. Phys.1999, 110, 7650 - 7657.