Potential cnergy surfaces for the reactions Ar'--;-HZ +ArH'+H and &+Hi -+ArH+t-H (ArH+ i n the ground state) are calculated by the method of diatoniics-in-molecules (DIM), the theory of which is conveniently summarized in matrix notation. Diatomic ArH+ and ArH curves needed for the method are obtained from ab initio SCFMO calculations. The shape of the ArHi surface appears to be consistent with the known experimental information for this system. Approximate calculations of collinear surfaces for the ground states of KrH: and NeHf, are obtained by appropriate scaIing of the ArH+ diatomic curves. Comparison of these surfaces with the ArHi surface results in a consistent rationalization of the available experimental data on the rare gas-hydrogen ionmolecule reactions. Gross differences in the behaviour of these reactions are related primarily to changes in the ionization potentials of the rare gas atoms. This has a profound effect on the shapes of the potential energy surfaces, particularly in the entrance valley, where there is a crossing between surfaces associated with the differently charged reactant species.
A recently developed systematic diatomics-in-molecules (DIM) procedure has been applied to the system H + H2 in order to generate large basis set models capable of approximating both the ground and low-lying excited state potential energy surfaces in a unified manner. The procedure, based exclusively on an analysis of diatomic ab initio wave functions, suggests that a 20-structure model including structures with not more than one excited H atom (2s or 2p) should suffice for the H3 (2 A ') states. An 80-structure model including up to two excited H atoms yielded potential energy surfaces in close agreement with the smaller model. The ground state surface shows a greatly improved behavior in D3h configurations when compared to the simplest, two-structure DIM model for H3 but is otherwise very similar to that surface. This result exemplifies the stability of our systematic DIM methodology to increases in the size of the basis set. A number of excited state surfaces, including the lower 2A " and quartet states, are reported and the implications for reaction kinetics are discussed. In particular, we predict the reaction H*(2s or 2p) + H 2 --H + H + H to have a large cross section.
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