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.