A COUPLED-CLUSTER STUDY OF THE GROUND-STATE OF C-3+

We report calculations of the structure and harmonic frequencies of the 2B2 state of C3+ and the energy of this state relative to the linear geometry (2-SIGMA-u+) using self-consistent-field (SCF) and coupled-cluster (CC) methods, including the full coupled-cluster single, double, and triple excitation (CCSDT) model. The calculations on 2B2 C3+ are the most complete treatment to date and may be of assistance in further experimental detection of this species. The calcualtions of the 2-SIGMA-u+ -2B2 energy difference support a bent structure. These calculations also show that, compared with full CCSDT and configuration interaction single, double, triple, and quadruple excitation (CISDTQ) results, CC methods which only approximately include effects of connected triple excitations seem for this example to give misleadingly small energy differences. A recent prediction by such approximate methods that C3+ may be quasilinear is therefore questionable. In the course of this work, certain practical difficulties in the SCF description of 2B2 C3+ were encountered, namely symmetry breaking of the restricted open-shell Hartree-Fock (ROHF) wave function and the existence of two distinct 2B2 unrestricted Hartree-Fock (UHF) solutions. We show that these can be alleviated by using our quasirestricted Hartree-Fock CC approach (QRHF-CC). Coupled-cluster single and double excitation (CCSD) calculations based on a QRHF reference function consisting of neutral molecule orbitals are able to provide all frequencies, unlike those based on a ROHF reference function. This work shows that QRHF-CC calculations offer a convenient single reference solution to certain problems involving symmetry breaking or other difficulties which traditionally have been solved by multireference methods. This quality of the QRHF-CC approach appears not to have been recognized previously.