The aluminum arsenides AlmAsn (m + n = 2–5) and their anions: Structures, electron affinities and vibrational frequencies

Abstract

Geometries, electronic states and electron affinities of AlmAsn and AlmAs n (m+n=2–5) clusters have been examined using four hybrid and pure density functional theory (DFT) methods. Structural optimization and frequency analyses are performed using a 6-311+G(2df) one-particle basis set. The geometries are fully optimized with each DFT method independently. The three types of energy separations reported in this work are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The calculation results show that the singlet structures have higher symmetry than that of doublet structures. The best functional for predicting molecular structures was found to be BLYP, while other functionals generally underestimated bond lengths. The largest adiabatic electron affinity, vertical electron affinity and vertical detachment energy, obtained at the 6-311+G(2df)/BP86 level of theory, are 2.20, 2.04 and 2.27 eV (AlAs), 2.13, 1.94 and 2.38 eV (AlAs2), 2.44, 2.39 and 2.47 eV (Al2As), 2.09, 1.80 and 2.53 eV (Al2As2), 2.01, 1.57 and 2.36 eV (AlAs3), 2.32, 2.11 and 2.55 eV (Al2As3), 2.40, 1.45 and 3.26 eV (AlAs4), 1.94, 1.90 and 2.07 eV (Al4As), respectively. However, the BHLYP method gives the largest values for EAad and EAvert of Al3As and EAad of Al3As2, respectively. For the vibrational frequencies of the AlnAsm series, the B3LYP method produces good predictions with the average error only about 10 cm-1 from available experimental and theoretical values. The other three functionals overestimate or underestimate the vibrational frequencies, with the worst predictions given by the BHLYP method.