Nonperturbative Many-body Treatment of Molecular Magnet Systems

Molecular magnets have seen significant attention due to their potential applications in quantum information / quantum computing. A delicate balance of electron correlation, spin-orbit coupling (SOC), ligand field splitting, and other effects produces a persistent magnetic moment within each molecular magnet unit. The competition among these effects poses a challenge for theoretical treatments. Electron correlation plays a central role since d-, or f-element ions provide the magnetic states in molecular magnets requiring explicit many-body treatments, in general beyond density functional theory (DFT). Current many-body treatments of molecular magnets rely on the perturbative inclusion of SOC, but this ignores feedback effects and can result in significant errors for strong SOC. Furthermore, molecular magnets are large systems, involving tens of atoms in even the smallest systems, and direct treatment is infeasible for most explicit many-body methods. Auxiliary-field quantum Monte Carlo (AFQMC) has demonstrated a high degree of reliability in correlated-electron systems with computational cost that scales as a low order polynomial. Recent advances in AFQMC technology allow an ab initio treatment in molecular magnets where electron correlation, SOC, and material specificity are included accurately and on an equal footing. We demonstrate the approach by applying AFQMC to compute the low energy spectrum of a linear Co(II) complex.