Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA.
J Chem Phys. 2012 Jan 28;136(4):044307. doi: 10.1063/1.3679014.
We report a high-quality, ab initio, full-dimensional global potential energy surface (PES) for the Cl((2)P, (2)P(3/2)) + CH(4) reaction, which describes both the abstraction (HCl + CH(3)) and substitution (H + CH(3)Cl) channels. The analytical PES is a least-squares fit, using a basis of permutationally invariant polynomials, to roughly 16,000 ab initio energy points, obtained by an efficient composite method, including counterpoise and spin-orbit corrections for the entrance channel. This composite method is shown to provide accuracy almost equal to all-electron CCSD(T)/aug-cc-pCVQZ results, but at much lower computational cost. Details of the PES, as well as additional high-level benchmark characterization of structures and energetics are reported. The PES has classical barrier heights of 2650 and 15,060 cm(-1) (relative to Cl((2)P(3/2)) + CH(4)(eq)), respectively, for the abstraction and substitution reactions, in good agreement with the corresponding new computed benchmark values, 2670 and 14,720 cm(-1). The PES also accurately describes the potential wells in the entrance and exit channels for the abstraction reaction. Quasiclassical trajectory calculations using the PES show that (a) the inclusion of the spin-orbit corrections in the PES decreases the cross sections by a factor of 1.5-2.5 at low collision energies (E(coll)); (b) at E(coll) ≈ 13,000 cm(-1) the substitution channel opens and the H/HCl ratio increases rapidly with E(coll); (c) the maximum impact parameter (b(max)) for the abstraction reaction is ~6 bohr; whereas b(max) is only ~2 bohr for the substitution; (d) the HCl and CH(3) products are mainly in the vibrational ground state even at very high E(coll); and (e) the HCl rotational distributions are cold, in excellent agreement with experiment at E(coll) = 1280 cm(-1).
我们报道了一个高质量的、从头算的、全维的 Cl((2)P, (2)P(3/2)) + CH(4) 反应的全局势能面（PES），它描述了均裂（HCl + CH(3)) 和取代（H + CH(3)Cl）两个反应通道。这个解析 PES 是通过使用置换不变多项式基的最小二乘法拟合得到的，它基于一个高效的组合方法，包括入口通道的平衡校正和自旋轨道校正，共使用了约 16000 个从头算能量点。这种组合方法被证明提供的精度几乎与全电子 CCSD(T)/aug-cc-pCVQZ 结果相当，但计算成本要低得多。我们报告了 PES 的详细信息，以及结构和能量的额外高精度基准特征。PES 对于均裂和取代反应的经典势垒高度分别为 2650 和 15060 cm(-1)（相对于 Cl((2)P(3/2)) + CH(4)(eq))，与新计算的基准值 2670 和 14720 cm(-1)吻合较好。PES 还准确地描述了均裂反应入口和出口通道中的势能阱。使用 PES 的准经典轨迹计算表明，（a）PES 中自旋轨道校正的包含将在低碰撞能（E(coll)）下使截面减小 1.5-2.5 倍；（b）在 E(coll) ≈ 13000 cm(-1)时，取代通道打开，H/HCl 比随 E(coll)迅速增加；（c）均裂反应的最大碰撞参数（b(max)）约为 6 bohr；而取代反应的 b(max)仅约为 2 bohr；（d）HCl 和 CH(3)产物主要处于振动基态，即使在非常高的 E(coll)下也是如此；（e）HCl 的转动分布很冷，与 E(coll) = 1280 cm(-1)时的实验结果非常吻合。
