The Spin-Flip Bethe-Salpeter Equation approach to transition metal dimers and defects in solids

The electronic structure of transition metal dimers is difficult to calculate for many computational methods due to strong correlation effects and open-shell configurations. The “spin flip” approach allows a single-reference approach to describe open-shell states as an excitation -- up or down in energy -- from a related single-reference high-spin state. The Spin-Flip Bethe Salpeter Equation (“SF-BSE”) approach provides an ab initio interaction kernel to describe these excitations. We show vertical excitation energies calculated in the SF-BSE approach for the diamond NV^- center, in good agreement with configuration interaction-based methods and experiment. The spin-flip approach even allows for an improved treatment of electronic correlation from the relevant electronic configurations. This allows for the calculation of ground- and excited-states of ferromagnetic ions of Mn₂ and Cr₂ dimers. Finally, SF-BSE can also be used to parameterize a simple Heisenberg model [Mayhall, et al. J Chem Phys, (2014)] for the antiferromagnetic Cr₂ dimer, which demonstrates promise for the method to be applied to more complex magnetic molecules.