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.