On the Computational Characterization of Charge-Transfer Effects in Noncovalently Bound Molecular Complexes
On the Computational Characterization of Charge-Transfer Effects in Noncovalently Bound Molecular Complexes
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DOI：
10.1021/acs.jctc.7b01256
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发表时间：
2018-05-01
影响因子：
5.5
通讯作者：
Head-Gordon, Martin
中科院分区：
文献类型：
--
作者：
Mao, Yuezhi;Ge, Qinghui;Head-Gordon, Martin
电荷转移(CT)是分子间络合物形成过程中的一种重要的结合力，已有多种理论模型被提出来量化这种影响。这些方法通常依赖于基于系统划分的“无CT”状态的定义，对于给定的分子间络合物，有时会产生显著不同的结果。本文详细研究了两种广泛使用的无CT态定义，即绝对定域分子轨道(ALMO)方法(只允许碎片上的轨道混合)和约束密度泛函理论(CDFT)方法(碎片电子布居是固定的)。文中还简要讨论了自然键轨道(NBO)和正则化对称适应微扰理论(SAPT)。ALMO和CDFT定义CT的结果在广泛的模型体系上进行了比较，包括氢键体系、硼烷络合物、金属羰基络合物以及水和金属阳离子形成的络合物。对于这些体系中的大多数，CDFT产生的平衡CT能量比基于ALMO的定义给出的要小得多。这主要是因为CDFT布居限制并不完全抑制CT，这意味着CDFT“无CT”状态实际上是CT污染的。这种污染的例子包括(I)匹配的向前和向后捐献(例如，甲酸二聚体)和(Ii)不改变碎片群体的单向CT。在水二聚体等系统中，除了轨道约束外，还采用3空间密度约束来量化后一种效应的大小。此外，绝热EDA表明，CT的几个可观察到的效应，如平面BH3分子与Lewis碱络合时的“金字塔化”，已经出现在“无CT”CDFT表面。这些结果揭示了ALMO和CDFT对CT的定义之间的本质区别，并表明前者更符合人们对CT在分子间结合中作用的公认理解。
Charge-transfer (CT) is an important binding force in the formation of intermolecular complexes, and there have been a variety of theoretical models proposed to quantify this effect. These approaches, which typically rely on a definition of a "CT-free" state based on a partition of the system, sometimes yield significantly different results for a given intermolecular complex. Two widely used definitions of the "CT-free" state, the absolutely localized molecular orbitals (ALMO) method (where only on-fragment orbital mixings are permitted) and the constrained density functional theory (CDFT) approach (where fragment electron populations are fixed), are carefully examined in this work. Natural bond orbital (NBO) and the regularized symmetry-adapted perturbation theory (SAPT) are also briefly considered. Results for the ALMO and CDFT definitions of CT are compared on a broad range of model systems, including hydrogen-bonding systems, borane complexes, metal carbonyl complexes, and complexes formed by water and metal cations. For most of these systems, CDFT yields a much smaller equilibrium CT energy compared to that given by the ALMO-based definition. This is mainly because the CDFT population constraint does not fully inhibit CT, which means that the CDFT "CT-free" state is in fact CT-contaminated. Examples of this contamination include (i) matching forward and backward donation (e.g., formic acid dimer) and (ii) unidirectional CT without changing fragment populations. The magnitude of the latter effect is quantified in systems such as the water dimer by employing a 3-space density constraint in addition to the orbital constraint. Furthermore, by means of the adiabatic EDA, it is shown that several observable effects of CT, such as the "pyramidalization" of the planar BH3 molecule upon the complexation with Lewis bases, already appear on the "CT-free" CDFT surface. These results reveal the essential distinctions between the ALMO and CDFT definitions of CT and suggest that the former is more consistent with accepted understanding of the role of CT in intermolecular binding.