An ab initio study of the Ar-HCN complex

The potential energy surfaces for the ground state of the Ar-HCN complex have been calculated at several levels of theory, including the single and double excitation coupled-cluster method with noniterative perturbational treatment of triple excitation CCSD(T). Calculations have been performed using the augmented correlation-consistent polarized triple zeta basis set supplemented with bond functions (aug-cc-pVTZ + bf). The global minimum with a well depth of approximately 141 cm(-1) has been found for the linear Ar-H-C-N geometry (Theta = 0.0 degrees) with the distance R between the Ar atom and the center of mass of the HCN molecule equal to 8.52a(0). In addition, the potential energy surface has been found to contain a long channel that extended from the bent configuration at R = 7.39a(0) and Theta = 59.7 degrees (a well depth of 126 cm(-1)) toward the T-shaped configuration with R = 7.16a(0) and Theta = 107.5 degrees (a well depth of 121 cm(-1)). The interaction energies have been analyzed using perturbation theory of intermolecular forces. The location of the global minimum is determined by the anisotropy of the dispersion and induction effects. The ground vibrational state dissociation energy D-0 determined by the collocation method has been found to be 105 cm(-1). The wave number of the Sigma(1) bend amounts to 4.2 cm(-1), somewhat below the experimental value (5.5 cm(-1)). (C) 1999 American Institute of Physics. [S0021-9606(99)30303-2].