Design Rules to Engineer the Spin Structure of Cr⁴⁺ Molecular Qubits via Matrix Modularity

Using a multilevel computational approach, we predict the zero-field splitting (ZFS) parameters of Cr(IV) molecular qubits with unprecedented accuracy, obtaining results in excellent agreement with experiments. We then apply the protocol to Cr(IV) molecular color centers embedded in non-isostructural tin host matrices. We show that by simply altering the matrix composition, one can effectively modify the relative energies of the spin sublevels, thus enabling a fine-tuning of the qubit's magnetic anisotropy for optimal performance in quantum technologies. We identify two effects influencing matrix design: the molecular symmetry of the qubit and the presence of inhomogeneous electrostatic fields arising from the chemical composition of the matrix. Finally, we compute spin-coherence times and discuss their sensitivity to the matrix environment through the ZFS parameters. Our work provides predictive strategies for tailoring the spin structure and coherence properties of molecular color centers through a rational control of their matrix environment.