Accurate and convergent energetics of color centers by wavefunction theory

The accurate description of in-gap states of point defects in semiconductors with significant multideterminant character presents a long-standing challenge for density functional theory (DFT) methods. In this study, we devise an ab initio methodology based on wavefunction theory (WFT) as a competing alternative approach. Specifically, we apply perturbation theory (NEVPT2) on top of a defect-localized many-body wavefunction (CASSCF). This quantum chemistry methodology, exemplified for the NV− center in diamond, is not only used for the calculation of energies and properties, but also for state-specific geometry optimization. By relaxing cluster models of increasing size and investigating convergence behavior, we accurately computed (i) the energy levels of NV− electronic states involved in the polarization cycle, (ii) the effect of Jahn-Teller distortion on measurable properties, (iii) the fine structure of ground and excited states, and (iv) the pressure dependence of zero-phonon lines. In addition, we predict hitherto uncharacterized high-lying excited states.