LARGE-SCALE AB-INITIO CALCULATIONS FOR C-3

Large-scale calculations by the single, double, and perturbative triple excitation coupled cluster [CCSD(T)] method have been carried out for C-3. A Linear equilibrium geometry with R(e) = 1.2945 Angstrom was obtained. The use of large basis sets and the inclusion of connected triple substitutions is crucial for an accurate representation of the extremely shallow bending potential. Rovibrational term energies were calculated from a three-dimensional CCSD(T) potential (basis: 177 contracted Gaussian-type orbitals) by the discrete variable representation (DVR). Compared with the best previous ab initio calculations [P. Jensen et al., J. Chem. Phys. 97, 3399 (1992)] agreement with experiment is improved by almost an order of magnitude. The errors in the wave numbers of the fundamentals are -7.7 (v(1)), 0.9 (v(2)), and 0.6 (v(3)) cm(-1), respectively.