CALCULATION OF THE COMPLETE-ACTIVE-SPACE SELF-CONSISTENT-FIELD POTENTIAL-ENERGY SURFACE, THE DIPOLE-MOMENT SURFACES, THE ROTATION VIBRATION ENERGIES, AND THE VIBRATIONAL TRANSITION MOMENTS FOR C3(X)OVER-TILDE1-SIGMA-G+

The present work reports an ab initio calculation of the potential-energy surface and corresponding dipole moment surfaces for the C3 molecule employing the full valence complete-active-space self-consistent-field method with a very large basis set. On the basis of the ab initio results we have calculated the rotation-vibration energies in the lower part of the energy spectrum with the MOR.BID (Morse oscillator rigid bender internal dynamics) method, and we find that the present ab initio potential-energy surface reproduces qualitatively the variation of the bending energy-level pattern with excitation of the stretching modes. We conclude from the ab initio results obtained in the present work, and from a fitting to experimental data carried out with the MORBID Computer program, that the equilibrium structure of C3 is linear. We have used the ab initio dipole moment calculated in the present work to compute the vibrational transition moments for selected vibrational transitions of C-12(3), obtaining transition moments of 0.44 and 0.35 D, respectively, for the nu-2 and nu-3 bands. The ab initio dipole moment values have further been used to calculate the line strengths, integrated absorption coefficients, and peak absorption coefficients of the low-J transitions in the nu-2 and nu-3 bands; the results of the theoretical intensity calculation are in keeping with recent experimental observations.