Linearized Pair-Density Functional Theory with Spin-Orbit Coupling.

We include spin-orbit coupling (SOC) effects in linearized pair-density functional theory (L-PDFT), which is a multistate 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. SO-L-PDFT removes the unphysical J-symmetry breaking observed in MC-PDFT. The accuracy of SO-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, and 13-17, the Ce3+ and U5+ ions, group 16 monohydrides, group 17 monoxides, lanthanide hexachlorides ([CeCl6]3-, [PrCl6]3-, and [NdCl6]3-), actinyl ions ([UO2]+, [NpO2]2+), and tricarbonatoactinyl complexes ([UO2(CO3)3]5-, [NpO2(CO3)3]4-). We also compare the results to new spin-orbit-inclusive calculations by single-state and multistate multireference perturbation theory.