Gaussian-3 Theory using Scaled Energies

The calculation of the binding energies of molecules to chemical accuracy (1 kcal/mol error) is an extremely challenging problem. It requires the inclusion of high levels of electron correlation and the use of large basis sets with very high angular momentum functions. Brute-force calculations employing such large basis sets are only possible for very small molecules. In order to make high accuracy possible for a wider variety of molecules, many successful theoretical models use correlated calculations with moderate sized basis sets and include a small number of molecule-independent parameters to correct for the remaining systematic deficiencies. For example, the successful G3 theory employs molecule-independent additive parameters ("higher-level corrections") that depend on the number of paired and unpaired electrons in the molecule to achive an accuracy of 1 kcal/mol for a large thermochemical test set. In this paper, a modification of Guassian-3 (G3) theory using multiplicative scale factors, instead of the additive higher level correction, is presented.