N-H vibrational frequencies in GaAs:N:H$_{n}$

While the fundamental H-related defect in GaAs:N and GaP:N appears to have an NH$_{2}$ character,[1] considerable recent evidence suggests that an additional one or two hydrogen atoms may in some cases be involved.[2,3] We have used the CRYSTAL06 \textit{ab} \textit{initio} quantum code[4] with density functional theory to investigate these possibilities, obtaining theoretical equilibrium positions, ground-state energies, and vibrational frequencies with all combinations of H and D. In all cases investigated, the NH$_{2}$ configuration is preserved, but observable shifts in the N-H or N-D vibrational frequencies are predicted. Supported by NSF Grant 541744. [1] S. Kleekajai \textit{et al.,} Phys. Rev. B \textbf{77}, 085213 (2008) and references contained therein. [2] A. A. Bonapasta \textit{et al.}, Phys. Rev. Lett. \textbf{98}, 206403 (2007). [3] M. Berti \textit{et al.}, Phys. Rev. B \textbf{76}, 205323 (2007). [4] R. Dovesi \textit{et al.}, \textit{Crystal06 User's Manual} (University of Torino, Torino, 2006).