Quantum Monte Carlo study of the oxygen vacancy in vanadium dioxide*

Transition metal oxides (TMO's) have presented fundamental challenges for accurate fully ab initio calculations. Vanadium dioxide, in particular, has remained difficult to describe in a consistent and predictive manner for leading theoretical approaches, such as density functional theory (DFT). Quantum Monte Carlo (QMC) methods present an attractive alternative to DFT as they have been shown to be accurate for a range of TMO's, including bulk phases of VO$_2$. In this work we expand the application of QMC to inquire into the behavior of isolated oxygen vacancies in VO$_2$, carefully controlling for known sources of systematic error. We compare our preliminary results with broadly used DFT approximations including hybrid functionals and Hubbard-U type approaches.