  We present an approach to calculation of point defect optical and thermal
ionization energies based on the highly accurate quantum Monte Carlo methods.
The use of an inherently many-body theory that directly treats electron
correlation offers many improvements over the typically-employed density
functional theory Kohn-Sham description. In particular, the use of quantum
Monte Carlo methods can help overcome the band gap problem and obviate the need
for ad-hoc corrections. We demonstrate our approach to the calculation of the
optical and thermal ionization energies of the F-center defect in magnesium
oxide, and obtain excellent agreement with experimental and/or other
high-accuracy computational results.
