四极流体中球平均相互作用与角相关相互作用
Spherically averaged versus angle-dependent interactions in quadrupolar fluids
摘要：
在计算机模拟中采用简化模型一方面通常受到计算机时间限制的限制，但另一方面它提供了对决定给定物理现象的分子性质的见解。我们采用这种策略来确定四极流体的相行为，在那里我们研究的影响，省略角自由度的分子通过一个有效的球对称电位从微扰膨胀。比较液-气共存曲线、共存时的蒸气压、共存相之间的界面张力等，如从具有完全四极相互作用和（近似）各向同性相互作用的模型获得的，我们发现临界区域中的差异通常（例如在二氧化碳的情况下）为4%的数量级。然而，当Lennard-Jones参数被重新标度，使得两个模型的临界温度和临界密度与实验结果一致时，两个模型的上述性质之间几乎完全一致。这一结果证明使用各向同性四极势是合理的。我们还提出了一个详细的比较，我们的模拟与积分方程/密度泛函方法相结合，并表明，后者提供了一个准确的描述，除了附近的临界点。
英文摘要：
Employing simplified models in computer simulation is on the one hand often enforced by computer time limitations but on the other hand it offers insights into the molecular properties determining a given physical phenomenon. We employ this strategy to the determination of the phase behaviour of quadrupolar fluids, where we study the influence of omitting angular degrees of freedom of molecules via an effective spherically symmetric potential obtained from a perturbative expansion. Comparing the liquid-vapor coexistence curve, vapor pressure at coexistence, interfacial tension between the coexisting phases, etc., as obtained from both the models with the full quadrupolar interactions and the (approximate) isotropic interactions, we find discrepancies in the critical region to be typically (such as in the case of carbon dioxide) of the order of 4%. However, when the Lennard-Jones parameters are rescaled such that critical temperatures and critical densities of both models coincide with the experimental results, almost perfect agreement between the above-mentioned properties of both models is obtained. This result justifies the use of isotropic quadrupolar potentials. We present also a detailed comparison of our simulations with a combined integral equation/density functional approach and show that the latter provides an accurate description except for the vicinity of the critical point.
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