The spin-orbit energy estimated from two-component spin-orbit calculations as correction terms for the Gaussian-2 (G2) theory

Abstract

Two-component relativistic effective core potential (RECP) approaches are employed to derive spin-orbit energies from the energy difference of spin-orbit two-component RECP and spin averaged RECP calculations. These spin-orbit energies include both the spin-orbit splitting of open-shell species and the higher order spin-orbit contributions for all species. The present spin-orbit energies, which can be estimated from the single reference configuration calculations at various levels of theory such as Hartree-Fock and coupled-cluster methods, are utilized as additional correction terms in the Gaussian-2 (G2) theory for Br and I containing species. The improvement in reaction energies due to the present scheme is similar to those reported for other types of spin-orbit calculations based on multireference configuration interaction calculations. The present scheme straightforwardly provides spin-orbit correction terms for the compounded methods such as the G2 theory. Even the spin-orbit corrections at the HF level are reliable enough to improve the performance of the G2 theory, reducing the error to the chemical accuracy (+/- 0.1 eV or 2 kcal mol(-1)) that is the target prescribed in G2 theory.