First principles study of polyatomic clusters of AlN, GaN, and InN. 1. Structure, stability, vibrations, and ionization

First principles calculations based on the nonlocal density approximation t o the density functional theory were performed to study structures, stabili ties, and vibrational properties of small (monomer, triatomic, and dimer) n eutral and ionized clusters of AlN, GaN, and InN. As a general trend, triat omic isomers prefer doubler spin states, whereas triplets are predicted for the monomer and the linear dimer clusters. Both nitrogen-excess and metal- excess triatomic clusters show minimum energy configurations to be approxim ately linear. The most stable isomer of Al2N2 and Ga2N2 is a rhombus with a singlet spin state, though In2N2 is predicted not to be stable against dis sociation into In-2 and N-2. A strong dominance of the N-N bond over the me tal-nitrogen and metal-metal bonds appears to control the structural skelet ons and the chemistry of these clusters. This is manifested in the dissocia tion of neutral and singly-ionized clusters, where the loss of metal atoms is shown to be the most likely fragmentation channel, except in the case of the dimer, in which the formation of two homonuclear diatomics is favored. The vibrational modes and frequencies are also explained in terms of the d ifferent bond strengths found in the diatomic clusters.