In some compounds, exchange repulsion is orientation dependent. However, in contrast to quantum chemical methods that treat exchange explicitly, empirical models assume exchange to be spherically symmetric, yielding an average description only. Here we quantify the anisotropy of exchange and dispersion energy for hydrogen halides and water by probing these compounds with a helium atom using the symmetry-adapted perturbation theory (SAPT). The exchange interaction is reduced by up to 33% due to the σ-hole in hydrogen iodide, depending on the location of the probe. We demonstrate how this anisotropy can be modeled in empirical force fields either using an angle-dependent potential or by introducing virtual sites, reducing the error in the empirical model by a factor of 5 compared to isotropic atoms. Lone-pairs on water, positioned close to perpendicular to the plane of the molecule, on a line with the oxygen atom, and, surprisingly, σ-holes on water both modulate the exchange interaction strongly. Both lone-pairs and σ-holes can be modeled by virtual sites, leading to an 80% reduced error.

中文翻译：

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交换和色散相互作用中各向异性的量化：基于物理的力场的简单模型


在一些化合物中，交换排​​斥取决于方向。然而，与明确处理交换的量子化学方法相反，经验模型假设交换是球对称的，仅产生平均描述。在这里，我们使用对称适应微扰理论（SAPT）用氦原子探测卤化氢和水的交换能和色散能的各向异性。由于碘化氢中的 σ 空穴，交换相互作用最多可减少 33%，具体取决于探针的位置。我们演示了如何使用角度相关势或通过引入虚拟位点在经验力场中对这种各向异性进行建模，与各向同性原子相比，将经验模型中的误差减少 5 倍。水上的孤电子对，位置接近垂直于分子平面，与氧原子在一条线上，令人惊讶的是，水上的 σ 空穴都强烈调节交换相互作用。孤对电子和 σ 空穴都可以通过虚拟站点进行建模，从而减少 80% 的误差。