A quantitative description of the excited electronic states of point defects and impurities is
crucial for understanding materials properties, and possible applications of defects in
quantum technologies. This is a considerable challenge for computational methods, since KohnSham density-functional theory (DFT) is inherently a ground state theory, while higher level methods are often too computationally expensive for defect systems. Recently, embedding
approaches have been applied that treat defect states with many-body methods, while using DFT
to describe the bulk host material. In this talk, I will discuss our implementation of such an embedding method, based on
Wannierization of defect orbitals and the constrained random-phase approximation.
I will review our systematic characterization of the method for three distinct systems with current
technological relevance: (i) a carbon dimer replacing a B and N pair in bulk hexagonal BN, (ii)
the negatively charged nitrogen-vacancy center in diamond (NV-), and (iii) an Fe impurity on
the Al site in wurtzite AlN (FeAl).
I will show that the embedding approach for CBCN gives many-body states in agreement with
analytical results on the Hubbard dimer model, which allows us to elucidate the effects of the
DFT functional and double-counting correction. For the NV- center, our method demonstrates
good quantitative agreement with experiments for the zero-phonon line of the triplet-triplet
transition. Finally, I will illustrate challenges associated with this method for determining the
energies and orderings of the complex spin multiplets in FeAl.
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