G3(MP2) Theory
G3(MP2) is a cheaper variant of G3 theory, in which the effects of basis set extension
are obtained from calculations at the MP2 level of theory. The G3(MP2) energy at
0 degrees Kelvin E0(G3MP2) is defined as:
The definition of the components being:
The necessary energies can be calculated most efficiently in the following sequence:
- Optimization and frequency calculation at the HF/6-31G(d) level of theory
- Optimization at the MP2(FULL)/6-31G(d) level of theory
- QCISD(T,FC)/6-31G(d)//MP2(FULL)/6-31G(d) single point
- MP2(FC)/G3MP2large//MP2(FULL)/6-31G(d) single point
Comments:
- Open shell systems are treated using unrestricted wavefunctions (UHF, UMP2 . . )
- The higher level correction (HLC) is supposed to compensate remaining deficiencies
of the method. In contrast to G2 theory, separate values for parameters A and B have been
optimized 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 G3MP2large basis set is almost identical to the G3large basis used in G3 except that
core polarization functions are not included. The G3MP2large basis sets can be found on the
G3 theory web site set up by Larry Curtiss.
A local copy can be found here.
- In contrast to G3 theory, the MP2 single point calculation is performed within the
frozen core approximation (MP2(FC)).
- The mean absolute devitiation for the extended G2 neutral set (148 reaction energies) is
1.18 kcal/mol.
Literature:
- L. A. Curtiss, K. Raghavachari,
"G2 Theory"
The Encyclopedia of Computational Chemistry, 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.
last changes: 01.04.2008, AS
questions & comments to: axel.schulz@uni-rostock.de