Model Chemistries

The combination of method and basis set specifies a model chemistry to Gaussian, specifying the level of theory. Every Gaussian job must specify both a method and basis set. This is usually accomplished via two separate keywords within the route section of the input file, although a few method keywords imply a choice of basis set. Some jobs using a density functional method may also include a density fitting set (see the Basis Sets section for more information).

The following table lists methods which are available in Gaussian, along with the job types for which each one may be used. An asterisk indicates analytic calculations, while numerical-only calculations are indicated by n (see the discussion of the specific keyword in question for details).

List of Methods and Their Available Job Types
List of Methods and Their Available Job Types

If no method keyword is specified, HF is assumed. Most method keywords may be prefaced by R for closed-shell restricted wavefunctions, U for unrestricted open-shell wavefunctions, or RO for restricted open-shell wavefunctions: for example, ROHF, UMP2, or RQCISD. RO is available only for Hartree-Fock and Density Functional methods, and AM1, PM3, PM3MM, PM6 and PDDG energies and gradients, and MP2, MP3, MP4, and CCSD energies.

In general, only a single method keyword should be specified, and including more than one of them will produce bizarre results. However, there are exceptions:


Last updated on: 9 Jun 2009