Unified large basis set diatomics-in-molecules models for ground and excited states of H3

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 (2A′) 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)+H2→H+H+H to have a large cross section.