Quantum Monte Carlo study of the optical and diffusive properties of the vacancy defect in diamond.

Fixed-node diffusion quantum Monte Carlo (DMC) calculations of the ground and excited state energetics of the neutral vacancy defect in diamond are reported. The multiplet structure of the defect is modeled using guiding wave functions of the Slater-Jastrow type with symmetrized multideterminant Slater parts. For the ground state we obtain the 1E state in agreement with experiment. The calculated energy of the lowest dipole allowed transition is consistent with the experimentally observed GR1 band, which has long been identified with the neutral vacancy in diamond, although no previous first-principles ab initio calculation of this transition exists. The calculated multiplet splitting of over 2 eV indicates the importance of a proper treatment of electron exchange and correlation in this system. DMC calculations of the formation and migration energy of the vacancy defect are presented.