AB-INITIO CALCULATIONS OF THE ROVIBRATIONAL STATES OF HE2C2+1

The low-lying rovibrational states of the dihelium carbene dication, H e2C2+, have been calculated using ab initio techniques. A 75-point pot ential energy surface was constructed using an all-electron coupled cl uster 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-H e bondlength of 1.570 Angstrom and a bond angle of 84.1 degrees. The d iscrete 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 pot ential function was embedded in the Eckart-Watson Hamiltonian, which w as solved variationally. The anharmonic fundamental frequencies for th e breathe, bend and asymmetric stretch vibrations were calculated to b e 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 minu s combinations of regular symmetric-top rotor functions. (C) 1997 Else vier Science B.V.