Choice of Spin-Orbit Correction Terms in Gaussian Model Chemistries

Spin-orbit correction terms for use in Gaussian-2 theory and other model chemistries for third-row atoms and molecules are calculated by several methods with the objective of finding a reliable method that can be applied in a routine and economical manner in the spirit of Gaussian model chemistries. The results are evaluated for the test set of molecules and ions used in the original extension of Gaussian-2 theory to third-row atoms. Further results are presented for systems where Gaussian-2 results are reported in the literature without spin-orbit correction terms and for some larger molecules.