Departament de Física Fonamental, Facultat de Física, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain.
J Chem Phys. 2010 Aug 28;133(8):084507. doi: 10.1063/1.3463424.
By molecular dynamic simulations we study a system of particles interacting through a continuous isotropic pairwise core-softened potential consisting of a repulsive shoulder and an attractive well. The model displays a phase diagram with three fluid phases: a gas-liquid critical point, a liquid-liquid critical point, and anomalies in density, diffusion, and structure. The hierarchy of the anomalies is the same as for water. Here we study in a systematic way the effect on the anomalies of varying the softness of the potential. We find that, making the soft-core steeper and more penetrable, the regions of density and diffusion anomalies contract in the T-rho plane, while the region of structural anomaly is weakly affected. Therefore, a liquid can have anomalous structural behavior without having density or diffusion anomalies. We show that, by considering as effective distances those corresponding to the maxima of the first two peaks of the radial distribution function g(r) in the high-density liquid, we can generalize to continuous two-scale potentials a criterion for the occurrence of the anomalies of density and diffusion, originally proposed for discontinuous potentials. However, we observe that the knowledge of the structural behavior within the first two coordination shells of the liquid is not enough to establish, in general, the occurrence of the anomalies. By introducing the density derivative of the cumulative order integral of the excess entropy, measuring shell by shell the amount of order in the liquid, we show that the anomalous behavior is regulated by the structural order at distances as large as the fourth coordination shell. By comparing the results for different softness of the potential, we conclude that the disappearance of the density and diffusion anomalies for the steeper potentials is due to a more structured short-range order. All these results increase our understanding on how, knowing the interaction potential, we can evaluate the possible presence of anomalies for a liquid.
通过分子动力学模拟，我们研究了一个由连续各向同性核-软势相互作用的粒子系统，该势由排斥肩部和吸引阱组成。该模型显示了具有三个流体相的相图：气-液临界点、液-液临界点以及密度、扩散和结构的异常。异常的层次结构与水相同。在这里，我们系统地研究了改变势的柔软度对异常的影响。我们发现，使软核更陡峭和更可穿透，密度和扩散异常区域在 T-rho 平面上收缩，而结构异常区域受到的影响较弱。因此，液体可以具有异常的结构行为而没有密度或扩散异常。我们表明，通过考虑作为有效距离，与高密液体中径向分布函数 g(r)的前两个峰的最大值相对应的距离，我们可以将密度和扩散异常的发生准则推广到连续的两尺度势，该准则最初是为不连续势提出的。然而，我们观察到，了解液体前两个配位壳层内的结构行为不足以普遍建立密度和扩散异常的发生。通过引入超额熵累积有序积分的密度导数，逐壳测量液体中的有序量，我们表明异常行为受第四配位壳层远处的结构有序调节。通过比较不同柔软度的结果，我们得出结论，对于更陡峭的势，密度和扩散异常的消失是由于更结构化的短程有序。所有这些结果都增加了我们对如何在知道相互作用势的情况下评估液体中异常可能存在的理解。
