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 C2v symmetry with an RC-He bondlength of 1.570 Å and a bond angle of 84.1 °. The discrete potential energy surface was fitted using a Padé (4, 5) power series expansion yielding a (x2)12 of 8.2 × 1−6 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.