  Paramagnetic point defects in wide-bandgap semiconductors, characterized by
atomic-like in-gap defect states, constitute a unique challenge for ab initio
modeling. In this theoretical study, we aim to devise a wave-function only
computational protocol, exemplified on the prominent nitrogen-vacancy (NV)
center in diamond, which enables the full characterization of future quantum
bit candidates implemented in color centers. We propose the application of the
second order $n$-electron valence state perturbation theory on top of the
complete active space self-consistent field approximation (CASSCF-NEVPT2) to
provide a balanced ab initio level description of the correlation effects
yielded from the defect orbitals and the embedding nanodiamond. By relaxing the
molecular cluster under the compression of the surrounding bulk material, we
manage to model both the vertical and relaxed experimental electronic spectra
within an average error margin of 0.1 eV. Furthermore, the experimentally
observed Jahn-Teller behavior of $^3E$ and $^1E$ states, the measured fine
structure of the triplet electronic states, as well as the expected
spin-selectivity are quantitatively reproduced by the presented methodology.
Our findings showcase that using conventional wave-function-based quantum
chemical approaches on carefully crafted cluster models can be a competing
alternative for discussing the energetics of point defects in solids.
