We present the results of ob inilio (SCF and CEPA) calculations of the interaction potentials for the Ci'P)-H2('Xl) system. The five potential curves corresponding to collinear (%,%) and orthogonal ('Az,'B,, 'BJ approaches of the C atom to the midpoint of the H, molecule have been computed, as well as the curve o f 'A' symmetry for an intermediate geometry. There results are analysed and used in quantum mechanical (closecoupling) calculations of the cross sections far excitation of the fine structure transitions in C('P,) by ortho-and para-H,. The rate coefficients for these transitions arc evaluated far a range o f temperatures encompassing those relevant to studies of the interstellar medium.

Ab initio calculations at the SCF and CEPA levels have been performed for the interaction potential between O(3P) and H2(X 1 Sigma g+) at separations in the van der Waals region. Potential curves for 3 Sigma -, 3 Pi (collinear approach), 3A2, 3B1, 3B2 (orthogonal approach) and 3A' (45' approach) are presented and used in quantum mechanical close coupling calculations of the cross sections for the de-excitation of fine structure transitions in O(3PJ) by ortho- and para-H2. The rate coefficients for these and the reverse transitions are evaluated for a wide range of temperatures.

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