The computation of diffusion coefficients in molecular systems ranks among the most useful applications of equilibrium molecular dynamics simulations. However, when dealing with the problem of fluid diffusion through vanishingly thin interfaces, classical techniques are not applicable. This is because the volume of space in which molecules diffuse is ill-defined. In such conditions, non-equilibrium techniques allow for the computation of transport coefficients per unit interface width, but their weak point lies in their inability to isolate the contribution of the different physical mechanisms prone to impact the flux of permeating molecules. In this work, we propose a simple and accurate method to compute the diffusional transport coefficient of a pure fluid through a planar interface from equilibrium molecular dynamics simulations, in the form of a diffusion coefficient per unit interface width. In order to demonstrate its validity and accuracy, we apply our method to the case study of a dilute gas diffusing through a smoothly repulsive single-layer porous solid. We believe this complementary technique can benefit to the interpretation of the results obtained on single-layer membranes by means of complex non-equilibrium methods.

中文翻译：

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通讯：一种通过平衡分子动力学模拟计算纯流体通过平面界面扩散的传输系数的方法

分子系统中扩散系数的计算属于平衡分子动力学模拟最有用的应用之一。但是，当通过稀薄的界面处理流体扩散问题时，经典技术不适用。这是因为分子扩散所在的空间的体积不确定。在这种情况下，非平衡技术可以计算出每单位界面宽度的传输系数，但是它们的弱点在于它们无法隔离易于影响渗透分子通量的不同物理机制的贡献。在这项工作中，我们提出了一种简单而准确的方法，通过平衡分子动力学模拟来计算纯流体通过平面界面的扩散传输系数，以每单位界面宽度的扩散系数的形式表示。为了证明其有效性和准确性，我们将我们的方法应用于通过平滑排斥单层多孔固体扩散的稀薄气体的案例研究。我们相信，这种互补技术可以有益于通过复杂的非平衡方法对单层膜获得的结果进行解释。