AB-INITIO POTENTIAL-ENERGY SURFACE AND VIBRATIONAL FREQUENCIES OF HCN

An ab initio quantum chemical study of the potential energy surface of hydrogen cyanide using MP2, ACPF, CCSD and CCSD(T) methods is reporte d, including an investigation of the importance of higher angular mome ntum functions. The surface obtained from the CCSD(T) calculation with a 5s, 4p, 3d, 2f; 4s, 3p, 2d! atomic natural orbital basis yields an equilibrium geometry and harmonic frequencies that are in excellent a greement with experiment, and predicts the fundamental band origins to within 10 cm-1 of the observed values. The calculated overtone and co mbination frequencies are compared with the available experimental dat a up to 19 000 cm-1.