Reduced Cost Computation\nand Exploitation of Accurate\nRadial Interaction Potentials for Barrier-Less Processes

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