BP Institute, Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge CB3 OEZ, United Kingdom.
J Chem Phys. 2009 Nov 28;131(20):204506. doi: 10.1063/1.3266845.
Molecular dynamics simulations have been used to compute physical properties of model fluids in which the particles interacted via the soft-sphere pair potential (SSP) phi(r)=epsilon(sigma/r)(n), where epsilon and sigma are the characteristic energy and distance, respectively. The emphasis is on small values of n, tending to the lower theromodynamically allowed bound of 3+. An accurate equation of state for the SSP fluid is obtained, consisting of two terms, and as n-->3+, the compressibility factor, Z tends to Z=B(2)zeta(n/3) for zeta>0, where B(2) is the second virial coefficient, and zeta=piNsigma(3)/6V is a nominal packing fraction for N particles in volume V. A simple formula for the position of the first peak in the radial distribution function in the soft particle limit is proposed and shown to agree with the simulation data. The fluid phase velocity autocorrelation function at fluid-solid coexistence becomes more oscillatory as n decreases. Values for the self-diffusion coefficient D and shear viscosity eta were calculated as a function of n and density, and these were used to estimate the n-dependence of an ideal glass transition. The glass transition shifts relatively further into the solid part of the phase diagram as softness ( approximately 1/n) increases. D decreases by ca. 75% and eta increases by about a factor of 3 along the fluid-solid coexistence line from n=infinity to 3.25. Non-Gaussian behavior was calculated from the particle displacements as a function of particle softness. A screened soft-sphere potential, SSSP, was introduced to explore the effects for small n of the long range part of the potential in relation to the scale of the local structure. The SSSP with suitable analytic form and parameters can give statistically indistinguishable results from the full SSP for the static properties, D and eta.
分子动力学模拟已被用于计算模型流体的物理性质，其中粒子通过软球对势（SSP）相互作用，势函数为(\phi(r)=\epsilon(\frac{\sigma}{r})^{n})，其中(\epsilon)和(\sigma)分别是特征能量和距离。重点关注(n)较小的值，趋向于热力学允许的下界 3+。得到了 SSP 流体的准确状态方程，由两个项组成，当(n\to3+)时，压缩因子(Z)趋于(Z=B(2)\zeta(n/3))，其中(\zeta>0)，(B(2))是第二维里系数，(\zeta=\frac{\pi N\sigma^{3}}{6V})是 N 个粒子在体积 V 中的名义堆积分数。提出了软粒子极限中径向分布函数第一峰位置的简单公式，并与模拟数据吻合。当(n)减小时，流体-固体共存时的流体相速度自相关函数变得更加振荡。根据(n)和密度计算了自扩散系数(D)和剪切粘度(\eta)的值，并利用这些值估计了理想玻璃转变的(n)依赖性。随着柔软度（约(\frac{1}{n})）的增加，玻璃转变相对更深入到相图的固体部分。从(n=\infty)到 3.25，沿流体-固体共存线，D 降低约 75%，(\eta)增加约 3 倍。从粒子位移作为粒子柔软度的函数计算了非高斯行为。引入了屏蔽软球势（SSSP），以研究在与局部结构尺度相关的势的长程部分对小(n)的影响。具有适当分析形式和参数的 SSSP 可以对静态特性、D 和(\eta)给出与全 SSP 统计上不可区分的结果。
