Munaò G, Costa D, Caccamo C
Dipartimento di Fisica, and Consorzio Nazionale Interuniversitario di Struttura della Materia, Università degli Studi di Messina-Contrada Papardo, 98166 Messina, Italy.
J Chem Phys. 2009 Apr 14;130(14):144504. doi: 10.1063/1.3098551.
We revisit the thermodynamic and structural properties of fluids of homonuclear hard dumbbells in the framework provided by the reference interaction site model (RISM) theory of molecular fluids. Besides the previously investigated Percus-Yevick (PY) approximation, we test the accuracy of other closures to the RISM equations, imported from the theory of simple fluids; specifically, we study the hypernetted chain (HNC), the modified HNC (MHNC) and, less extensively, the Verlet approximations. We implement our approach for models characterized by several different elongations, up to the case of tangent diatomics, and investigate the whole fluid density range. The theoretical predictions are assessed against Monte Carlo simulations, either available from literature or newly generated by us. The HNC and PY equations of state, calculated via different routes, share on the whole the same level of accuracy. The MHNC is applied by enforcing an internal thermodynamic consistency constraint, leading to good predictions for the equation of state as the elongation of the dumbbell increases. As for the radial distribution function, the MHNC appears superior to other theories, especially for tangent diatomics in the high density limit; the PY approximation is better than the HNC and Verlet closures in the high density or elongation regime. Our structural analysis is supplemented by an accurate inversion procedure to reconstruct from Monte Carlo data and RISM the "exact" direct correlation function. In agreement with such calculations and consistent with the forecast of rigorous diagrammatic analysis, all theories predict the occurrence in the direct correlation function of a first cusp inside the dumbbell core and (with the obvious exception of the PY) of a second cusp outside; the cusps' heights are also qualitatively well reproduced by the theories, except at high densities.
我们在分子流体的参考相互作用位点模型（RISM）理论所提供的框架内，重新审视同核硬哑铃流体的热力学和结构性质。除了之前研究过的珀西 - 耶维克（PY）近似之外，我们还测试了从简单流体理论引入的其他RISM方程封闭近似的准确性；具体而言，我们研究了超网链（HNC）、修正超网链（MHNC），以及较少涉及的维里近似。我们针对具有几种不同伸长率的模型实施我们的方法，直至相切双原子分子的情况，并研究整个流体密度范围。理论预测通过与蒙特卡罗模拟进行对比评估，这些模拟数据要么来自文献，要么是我们新生成的。通过不同途径计算得到的HNC和PY状态方程在整体上具有相同的准确度。MHNC通过强制施加内部热力学一致性约束来应用，随着哑铃伸长率的增加，对状态方程能给出良好的预测。至于径向分布函数，MHNC似乎优于其他理论，特别是在高密度极限下对于相切双原子分子的情况；在高密度或伸长率区域，PY近似比HNC和维里封闭近似更好。我们的结构分析通过一种精确的反演程序得到补充，以便从蒙特卡罗数据和RISM重建“精确”的直接相关函数。与这些计算结果一致，并与严格的图解分析预测相符，所有理论都预测在直接相关函数中，哑铃核内部会出现第一个尖点，并且（PY明显除外）在外部会出现第二个尖点；除了在高密度情况下，尖点的高度在定性上也能被这些理论很好地再现。
