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Interpolated ab initio potential energy surfaces which describe abstraction and exchange reactions in collisions of hydrogen and water are reported. The electronic structure calculations are performed at the QCISD(T) level of theory, with an additivity approximation. A sufficiently large basis set is required to describe the Rydberg character of the electronic state for molecular configurations which are important for the exchange process. Classical and quantum dynamics calculations on the surfaces are presented. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: The CH3 + +H2 →CH5 + + radiative associative reaction plays a major role in the synthesis of complex hydrocarbons in interstellar dust clouds and was studied using a part-classical, part-ab initio approach. A first-principles potential energy surface (PES) at CCSD/6-311++G(2df, 2pd) was constructed from a set of 828 data points in configurational space using modified Shepard interpolation, and was used in conjuction with long-range perturbation theory at large intermolecular distances. Data points are chosen so as to optimize PES construction according to the GROW software algorithm. Classical reaction dynamics was employed to infer the reaction rate coefficient. [Copyright &y& Elsevier]