Linearized Pair-Density Functional Theory with Spin-Orbit Coupling

We present the inclusion of spin-orbit coupling (SOC) effects in linearized pair-density functional theory (L-PDFT), which is a multi-state extension of multiconfiguration pair-density functional theory (MC-PDFT). Both 1-electron and 2-electron SOC integrals are computed using Breit-Pauli and Douglas-Kroll-Hess Hamiltonians in the atomic mean-field approximation. SOC-L-PDFT removes the unphysical the J-symmetry breaking observed in MC-PDFT. The accuracy SOC-L-PDFT is validated by calculations of zero-field splittings, fine-structure excitation energies, and low-energy excited-state spectra for a diverse group of atoms and molecules spanning the whole range of the periodic table, including atoms of groups 3, 11, 13-17, the Ce3+ and U5+ ions, group 16 monohydrides, group 17 monoxides, lanthanide hexachlorides, actinyl ions and tricarbonatoactinyl complexes. We also compare the results to new spin-orbit-inclusive calculations by single-state and multi-state multireference perturbation theory