Relativistic Two-Component Multireference Configuration Interaction Method with Tunable Correlation Space

The multiconfiguration nature of late-row (≥4th) elements and their molecular complexes, combined with significant relativistic effects, present large challenges for the accurate description of their electronic structure. To address these challenges and incorporate both relativistic and electron correlation effects, we present a two-component Kramers-unrestricted multireference configuration interaction method where relativistic effects are included variationally at the molecular orbital level via use of the "exact two-component" transformation of the solution of the one-electron modified Dirac equation. This method is developed within the restricted active space framework, allowing flexibility in both the choice of correlation space and the level of truncation of the excitation operator, as well as promoting the efficiency of generating and book-keeping unique electronic configurations. This method is applied to the study of fine structure splitting in selected p-block and d-block elements and is further applied to the study of the open-shell heavy-element uranium(V) ion.