Ab initio calculations of the rovibrational states of He2C2+1

The low-lying rovibrational states of the dihelium carbene dication, He2C2+, have been calculated using ab initio techniques. A 75-point potential energy surface was constructed using an all-electron coupled cluster single, double and triple excitation (CCSD (T)) method together with a correlation-consistent polarised core valence triple-zeta basis set. The CCSD(T) optimised geometry was of C-2, symmetry with an RC-He bondlength of 1.570 Angstrom and a bond angle of 84.1 degrees. The discrete potential energy surface was fitted using a Pade (4, 5) power series expansion yielding a (chi(2))(1/2) of 8.2 x 10(-6) a.u. The potential function was embedded in the Eckart-Watson Hamiltonian, which was solved variationally. The anharmonic fundamental frequencies for the breathe, bend and asymmetric stretch vibrations were calculated to be 683.1, 329.5, 623.8 cm(-1) respectively. The low-lying rovibrational states were calculated variationally using a 560-configuration basis involving products of the vibrational eigenfunctions and plus and minus combinations of regular symmetric-top rotor functions. (C) 1997 Elsevier Science B.V.