Designing Molecular Qubits: Computational Insights into First-Row and Group 6 Transition Metal Complexes

The discovery of novel molecular systems for quantum information processing remains a major challenge for advancing molecular qubit technology. In the realm of optically addressable systems, a previously synthesized and characterized Cr(IV) pseudo-tetrahedral complex, featuring four strongly donating o-tolyl ligands surrounding the chromium center, has demonstrated potential as a qubit candidate. This study proposes analogs of this complex through a metal substitution strategy, extending the investigation to new complexes. The new metal centers were selected from first-row and Group 6 transition metals. Multiconfigurational methods such as the complete active space followed by second-order perturbation theory and multiconfiguration pair-density functional theory were utilized to calculate energy gaps between electronic states with different spins and zero-field splitting (ZFS). The results corroborate previous experimental findings for the Cr complex. Additionally, this study identifies a previously unsynthesized Ti(II) compound as a promising candidate for molecular qubits. This finding highlights the role of computational multireference methods in the rational design of qubit systems.