Ab initiotreatment of silicon defect clusters. The unrelaxed, neutral monovacancy

An embedding cluster theory is used to treat the neutral-vacancy defect in silicon by ab initio methods. Low-lying multiples associated with the defect electrons are calculated by using accurate configuration-interaction expansions of the many-electron wave functions that include coupling to virtual (unoccupied) and bulk valence orbitals. It is found that for the nuclei in their unrelaxed positions, correlation effects are necessary to give the correct ordering among the fully covalent states: ${}_{}{}^1{}_{}{}^{}E\left(0.0\mathrm{} \mathrm{eV}\right)<\mathrm{}{}_{}{}^3{}_{}{}^{}T_1^{}{}_{}{}^{}\left(0.1\mathrm{} \mathrm{eV}\right)<\mathrm{}{}_{}{}^5{}_{}{}^{}A_2^{}{}_{}{}^{}\left(0.7\mathrm{} \mathrm{eV}\right)$. The partially ionic states ${}_{}{}^1{}_{}{}^{}T_2^{}{}_{}{}^{}\left(3.6\mathrm{} \mathrm{eV}\right)<\mathrm{}{}_{}{}^1{}_{}{}^{}A_1^{}{}_{}{}^{}\left(5.0\mathrm{} \mathrm{eV}\right)$ are, in contrast, not sensitive to correlation corrections relative to the lower states.