Rotational excitation of C 2 (X 1 Σ g + ) by para - and ortho -H 2 .

A new four dimensional (4D) potential energy surface for the C 2 (X 1 Σ g + )-H 2 van der Waals system is generated. The potential was obtained from a multi-reference internally contracted configuration-interaction method including the Davidson correction (MRCI+Q). The four atoms were described using the augmented correlation-consistent quadruple zeta (aug-cc-pVQZ) basis sets. Both molecules were treated as rigid rotors. Close-coupling calculations of the inelastic integral cross sections of C 2 in collisions with para -H 2 ( j H 2 = 0) and ortho -H 2 ( j H 2 = 1) were also carried out at low energies. After Boltzmann thermal averaging, rate coefficients were obtained for temperatures ranging from 5 to 100 K. The rate coefficients for collisions with ortho -H 2 are significantly larger than the rate coefficients for collisions with para -H 2 .