In C_{2v symmetry, the ground and low-lying excited states of aluminum cyanide (AlCN) and its ions have been studied by using the complete active space self-consistent field (CASSCF) and multiconfigurational second-order perturbation theory (CASPT2) methods. For the low-lying electronic states of AlCN and its ions, the geometries, leading configurations and configuration interaction coefficients, harmonic vibrational frequencies, adiabatic excitation energies, vertical excitation energies and oscillator strengths are calculated with the ANO-L basis sets. At the CASSCF/CASPT2 level, for the 1¹Π electronic state of AlCN, the adiabatic excitation energy is calculated to be 352.4 kJ•mol^－1, which agrees well with the experimental value of 344.0 kJ•mol^－1. In addition, the transition X¹Σ^＋→2¹Π of AlCN is predicted at 673.1 kJ•mol^－1. The calculations of excitation energies and oscillator strengths provide a theoretical guidance to the investigation on absorption spectrum of AlCN.}

Key words: aluminum cyanide, the excited state, complete active space self-consistent field, multiconfigurational second-order perturbation theory