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.

中文翻译：

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降低成本计算和利用精确的径向相互作用电位实现无障碍过程


无障碍步骤是气相中自由基和离子反应的典型表现，通常发生在极端环境中，例如在非常高温度下运行的燃烧反应器或以超低温和压力为特征的星际介质。在模拟这些环境的条件下进行实验研究的难度表明，计算方法可以提供有价值的支持。在这方面，在过渡态理论框架中对这些过程的最先进处理能够提供准确的动力学参数，前提是底层势能表面足够准确。由于这需要平衡处理静态和动态相关性（在不同区域起着不同的作用），因此需要非常复杂且昂贵的量子化学方法。这个问题的一个有效解决方案是通过计算精确的一维径向电位来提供的，然后用它来校正由更便宜的量子化学方法执行的蒙特卡洛采样的结果。在本文中，我们将表明，对于一大组不同的无势垒反应步骤，径向电位由 ^R-4 项决定，并且添加进一步的 ^R-6 贡献可提供与参考点的定量一致性。这个结果的后果并非微不足道，因为参考电位可以通过非常有限的点（可能具有非线性间距）进行拟合。在由长程过渡态决定的反应步骤的情况下，还给出了在相空间理论框架中计算反应速率的广义表达式。 所有这些改进都为计算涉及大分子的无屏障反应的反应速率铺平了道路。