Structure and Excitation Spectra of Third-Row Transition Metal Hexafluorides Based on Multi-Reference Exact Two-Component Theory

The structures and some vertical excitation energies of third-row transition metal hexafluorides (MF₆, M = Re, Os, Ir, Pt, Au, Hg) were calculated using the generalized-active-space configuration interaction (GASCI) theory based on the exact two-component (X2C) Hamiltonian. The spin–orbit coupling (SOC) was included at the Hartree–Fock level, enabling us to analyze the SOC at the orbital level (spinor-representation). The excitation spectra were assigned based on the double group, a relativistic group theory applicable to states with the SOC. This study provides a fundamental understanding of the ligand field splitting, including the SOC, that is useful for the photochemistry and spin chemistry involving heavy elements.