Reduced Cost Computation and Exploitation of Accurate Radial Interaction Potentials for Barrier-Less Processes

Barrier-less steps are typical of radical and ionic reactions in the gas-phase, which often take place in extreme environments such as the combustion reactors operating at very high temperatures or the interstellar medium, characterized by ultralow temperatures and pressures. The difficulty of experimental studies in conditions mimicking these environments suggests that computational approaches can provide a valuable support. In this connection, the most advanced treatments of these processes in the framework of transition state theory are able to deliver accurate kinetic parameters provided that the underlying potential energy surface is sufficiently accurate. Since this requires a balanced treatment of static and dynamic correlation (which play different roles in different regions), very sophisticated and expensive quantum chemical approaches are required. One effective solution of this problem is offered by the computation of accurate one-dimensional radial potentials, which are then used to correct the results of a Monte Carlo sampling performed by cheaper quantum chemical approaches. In this paper, we will show that, for a large panel of different barrier-less reaction steps, the radial potential is ruled by the R^–4 term and that addition of a further R^–6 contribution provides quantitative agreement with the reference points. The consequences of this outcome are not trivial, since the reference potential can be fitted by a very limited number of points possibly with a nonlinear spacing. In the case of reaction steps ruled by long-range transition states, generalized expressions are also given for computing reaction rates in the framework of the phase-space theory. All these improvements pave the way toward the computation of reaction rates for barrier-less reactions involving large molecules.