An open-shell Hartree–Fock (HF) theory for spin-dependent two-component relativistic calculations, termed the Kramers-restricted open-shell HF (KROHF) method, is developed. The present KROHF method is defined as a relativistic analogue of ROHF using time-reversal symmetry and quaternion algebra, based on the Kramers-unrestricted HF (KUHF) theory reported in our previous study (Int. J. Quantum Chem., doi:10.1002/qua.25356). As seen in the nonrelativistic ROHF theory, the ambiguity of the KROHF Fock operator gives physically meaningless spinor energies. To avoid this problem, the canonical parametrization of KROHF to satisfy Koopmans' theorem is also discussed based on the procedure proposed by Plakhutin et al. (J. Chem. Phys. 2006, 125, 204110). Numerical assessments confirmed that KROHF using Plakhutin's canonicalization procedure correctly gives physical spinor energies within the frozen-orbital approximation under spin–orbit interactions.
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