Configuration vibronic mixing for a neutral vacancy in silicon and diamond

Configuration vibronic mixing is considered for a fully symmetric Jahn-Teller electronic term with orientation-degenerate terms (due to the distortion direction) including a correlation correction in a single-open-shell approximation. The approach is nonempirical and involves only linear vibronic coupling. The adiabatic potential is a multiwell one, because the different configurations involved in the exact Jahn-Teller term have different vibronic coupling with a lattice distortion. The stabilization energy, the frequencies of local lattice vibrations, the vibronic coupling parameter, and the energy barriers to migration and to distortion-axis reorientation are estimated for a neutral vacancy in silicon and diamond with allowance made for configuration vibronic coupling. The estimates agree with the results obtained by different experimental and theoretical methods for a wide range of properties associated with the Jahn-Teller effect.