Transition Metal Spin-State Energetics by MC-PDFT with High Local Exchange

The energetics of spin states of transition metal complexes have been explored with a variety of electronic structure methods, but the calculations require a compromise between accuracy and affordability. In this work, the spin splittings of several iron complexes are studied with multiconfigurational pair-density functional theory (MC-PDFT), and the results are compared to previously published results obtained by complete active space second-order perturbation theory (CASPT2) and CASPT2 with coupled-cluster semi-core correlation (CASPT2/CC). In contrast to CASPT2’s systematic overstabilization of high-spin states with respect to the CASPT2/CC reference, MC-PDFT with the tPBE on-top functional understabilizes high-spin states. The systematic understabilization is largely corrected by revising the exchange and correlation contributions to the on-top functional using the high local exchange approximation (tPBE-HLE). Moreover, tPBE-HLE correctly predicts the spin of the ground state in most cases, while CASP...