Four-component relativistic configuration-interaction calculation using the reduced frozen-core approximation

The reduced frozen-core approximation (RFCA) is a kind of frozen-core approximation (FCA). In our previous study [J. Chem. Phys. 109, 8182 (1998)], we found that Dirac–Fock–Roothaan calculations using RFCA had some desirable features compared to the all-electron and usual FCA calculations. In this study, in addition to these features, we found that they also describe well unoccupied as well as occupied orbitals. We developed a fully relativistic multireference configuration-interaction (CI) program that incorporates these features of RFCA, and report CI calculations for ThO and AuH. The calculated spectroscopic constants agree well with experimental values.