Multiconfiguration Pair-Density Functional Theory for Chromium(IV) Molecular Qubits

Pseudo-tetrahedral organometallic complexes containing chromium(IV) and aryl ligands have been experimentally identified as promising molecular qubit candidates. Here we present a computational protocol based on multiconfiguration pair-density functional theory for computing singlet-triplet gaps and zero-field splitting (ZFS) parameters in Cr(IV) aryl complexes. Notably, complete active space second-order perturbation theory (CASPT2) and hybrid multiconfiguration pair-density functional theory (HMC-PDFT) perform better than Kohn-Sham density functional theory for singlet-triplet gaps. Despite the very small values of the ZFS parameters, all the examined multiconfigurational methods performed qualitatively well. CASPT2 and HMC-PDFT performed particularly well for predicting the trend in the ratio of the rhombic and axial ZFS parameters, |E/D|. We have also investigated the dependence and sensitivity of the calculated ZFS parameters on the active space and the procedure for geometry optimization. The methodologies outlined here will guide future prediction of ZFS parameters in molecular qubit candidates.