Vertical excitation energies of the SiV(0) defect in diamond computed by the Spin-Flip Bethe-Salpeter Equation approach

The neutral silicon-vacancy defect in diamond (“SiV(0)”) is a promising candidate for qubit and nanosensing applications, and unlike the well-studied NV^- center, possesses inversion symmetry. As a potential qubit, optical transition energies from the defect’s triplet ground state to its triplet excited states are important quantities to calculate. Compared to the NV^- center, however, there are fewer calculations of these transition energies. As the defect has an open-shell electronic structure, computational methods must capture the multiconfigurational nature of its ground and excited states. We present calculations of singlet and triplet vertical excitation energies (that is, relative to the ground state energy at the ground state atomic coordinates) computed via the Spin-Flip Bethe-Salpeter Equation approach (“SF-BSE,” arXiv:2207.04549). SF-BSE is a method based on many-body perturbation theory that allows for the simultaneous computation of ground and excited state energies, from a basis of “target” states constructed from exciting an occupied up-spin electron to an unoccupied down-spin empty orbital. While some excited states for the NV^- defect have contributions from double-excitations that are inaccessible via single-reference spin-flip methods, the excited states of interest for SiV(0) do not, allowing for improved quantitative agreement for vertical excitation energies. Results from this approach are compared to other multireference methods in the literature.