An accurate analytic H4 potential energy surface
An accurate analytic H4 potential energy surface
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DOI：
10.1063/1.1405008
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发表时间：
2002-01-08
影响因子：
4.4
通讯作者：
Peterson, MJ
中科院分区：
文献类型：
--
作者：
Boothroyd, AI;Martin, PG;Peterson, MJ
H-4 的相互作用势能面 (PES) 作为分子间相互作用的测试用例对于量子化学非常重要。还需要详细了解某些天体物理过程，即分子云中 H-2 的碰撞激发和解离，其密度太低而无法通过实验进行。 Boothroyd 于 1991 年报道的 6101 从头计算 H-4 能量表明，当时可用的分析 H-4 表面存在很大的不准确性。 Keogh 和 Aguado 拟合这些能量的更精确的 H-4 表面中仍然存在一些不需要的特征，部分原因是 6101 从头能量提供的六维 H-4 构象空间的覆盖范围相对稀疏。为了提高覆盖范围，使用 Buenker 的多参考（单和）双激发配置交互程序计算了 42 079 个新的从头算 H-4 能量。这里计算了最低激发态和基态，并重新计算了原始 6101 构象的能量。从头算能量的估计均方根“随机”误差类似于 0.5 毫哈特里，系统误差类似于 1 毫哈特里 (0.6 kcal/mol)。新的分析 H-4 PES 适用于这 48 180 个从头计算能量（以及大间距下生成的额外 13 367 个点），与之前的 H-4 表面相比有了显着改进。这个新的 PES 相对于这 48 180 从头能量有 1.43 毫哈特里的均方根误差（拟合程序使用了高能量的减少权重，对于这 48 180 从头能量产生了 1.15 毫哈特里的加权均方根误差）。对于低于 H-2 解离能两倍的 39 064 ab initio 能量，新 PES 的均方根误差为 0.95 毫哈特里。这些均方根误差与从头算能量本身的估计误差相当。新的 PES 也很好地拟合了范德华，准确度约为 5%。对于相对紧凑的构象（能量高于 H-2 解离能），基态和第一激发态之间的圆锥形相交是分析表面中最大的误差源。这个圆锥相交的位置在H-4的六维构象空间中形成了一个有点复杂的三维超曲面。圆锥形交叉点的大部分位置已经被绘制出来，但是试图将圆锥形交叉点明确地包含在分析表面中超出了本文的范围。 (C) 2002 年美国物理研究所。
The interaction potential energy surface (PES) of H-4 is of great importance for quantum chemistry as a test case for molecule-molecule interactions. It is also required for a detailed understanding of certain astrophysical processes, namely collisional excitation and dissociation of H-2 in molecular clouds, at densities too low to be accessible experimentally. The 6101 ab initio H-4 energies reported in 1991 by Boothroyd demonstrated large inaccuracies in analytic H-4 surfaces available at that time. Some undesirable features remained in the more accurate H-4 surfaces fitted to these energies by Keogh and by Aguado , due in part to the relatively sparse coverage of the six-dimensional H-4 conformation space afforded by the 6101 ab initio energies. To improve the coverage, 42 079 new ab initio H-4 energies were calculated, using Buenker's multiple reference (single and) double excitation configuration interaction program. Here the lowest excited states were computed as well as the ground state, and energies for the original 6101 conformations were recomputed. The ab initio energies have an estimated rms "random" error of similar to0.5 millihartree and a systematic error of similar to1 millihartree (0.6 kcal/mol). A new analytical H-4 PES was fitted to these 48 180 ab initio energies (and to an additional 13 367 points generated at large separations), yielding a significant improvement over previous H-4 surfaces. This new PES has an rms error of 1.43 millihartree relative to these 48 180 ab initio energies (the fitting procedure used a reduced weight for high energies, yielding a weighted rms error of 1.15 millihartree for these 48 180 ab initio energies). For the 39 064 ab initio energies that lie below twice the H-2 dissociation energy, the new PES has an rms error of 0.95 millihartree. These rms errors are comparable to the estimated error in the ab initio energies themselves. The new PES also fits the van der Waals well to an accuracy of about 5%. For relatively compact conformations (energies higher than the H-2 dissociation energy), the conical intersection between the ground state and the first excited state is the largest source of error in the analytic surface. The position of this conical intersection forms a somewhat complicated three-dimensional hypersurface in the six-dimensional conformation space of H-4. A large portion of the position of the conical intersection has been mapped out, but trying to include the conical intersection explicitly in an analytic surface is beyond the scope of the present paper. (C) 2002 American Institute of Physics.