Gaussian-2 Theory

If the energy of a molecule at its equilibrium geometry is all that is required, then a semi-empirical method for determining the correlation energy is available. Gaussian-2 (G2) theory [50,51,52,53] is a composite procedure, using HF, MP2, MP4 and QCISD(T) (therefore scaling as O(N⁷)). It has been parametrized using 125 experimentally well-characterized atomization energies, ionization energies, electron affinities and proton affinities.

The procedure approximates a large basis QCISD(T) calculation using a series of additive approximations. G2 theory is usually within 1-2 kcal/mol of experiment. This combined with its `black box' nature is making the method very popular -- even for correcting experimental energies. However, the method has been shown to fail for some quite simple problems [54].