Overview of Gaussian Theories

G3 is an improvement over G2 theory in several ways and tries to reproduce effectively QCISD(T)/G3large

energies through a series of calculations at lower level. The G3large basis set is a slightly modified

version of the 6-311+G(3df,2p) basis set used in the G2 procedure. The G3 energy at 0 degrees Kelvin

E

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
- MP4(FC)/6-31+G(d)//MP2(FULL)/6-31G(d) single point
- MP4(FC)/6-31G(2df,p)//MP2(FULL)/6-31G(d) single point
- MP2(Full)/G3large//MP2(FULL)/6-31G(d) single point

- While a 6-311+G(3df) basis set has been used in G2 theory for all first and second row

elements, a different strategy has been pursued in G3 theory. First, a slightly smaller

6-311+G(2df) basis has been chosen for the first row elements, while a larger 6-311+G(3d2f)

basis has been chosen for the second row elements. In both cases core polarization functions

(e.g. p- and d-type for carbon) have also been added. Hydrogen is still treated with a 311+G(2p)

basis. The G3large basis sets can be found on the G3 theory web site set up by Larry Curtiss.

A local copy can be found here. - 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.

Overview of Gaussian Theories

Questions & Comments to:sulee@hanyang.ac.kr