Keyes T, Chowdhary J
Department of Chemistry, Boston University, Boston, Massachusetts 02215, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2002 Apr;65(4 Pt 1):041106. doi: 10.1103/PhysRevE.65.041106. Epub 2002 Apr 3.
The mechanism of diffusion in supercooled liquids is investigated from the potential energy landscape point of view, with emphasis on the crossover from high- to low-T dynamics over the range T(A) > or =T > or =T(c). Molecular dynamics simulations with a time dependent mapping to the associated local minimum or inherent structure (IS) are performed on unit-density Lennard-Jones. Dynamical quantities introduced include r2(is)(t), the mean-square displacement (MSD) within a basin of attraction of an IS, R2(t), the MSD of the IS itself, and g(t), the distribution of IS waiting times. The configuration space is treated as a composite of the contributions of cooperative local regions, and a method is given to obtain the physically meaningful g(loc)(t) and mean waiting time tau(loc) from g(t). An understanding of the crossover is obtained in terms of r2(is)(t) and tau(loc). At intermediate T, r2(is)(t) possesses an interval of linear t dependence allowing calculation of an intrabasin diffusion constant D(is). Near T(c), where intrabasin diffusion is well established for t<tau(loc), diffusion is intrabasin dominated with D=D(is); D may be calculated within a basin. Below T(c), tau(loc) exceeds the time tau(pl) needed for the system to explore the basin, indicating the action of barriers at the border; tau(loc)=tau(pl) is a criterion for transition to activated hopping. Intrabasin diffusion provides a means of confinement not involving barriers and plays a key role in the dynamics above T(c). The distinction is discussed between motion among the IS (IS dynamics) below T(c) and saddle or border dynamics above T(c), where the system is always close to one of the saddle barriers connecting the basins and IS boundaries are closely spaced and easily crossed. A border index is introduced based upon the relation of R2(t) to the conventional MSD, and shown to vanish at T approximately T(c). It is proposed that intrabasin diffusion is a manifestation of saddle dynamics.
从势能景观的角度研究了过冷液体中的扩散机制，重点关注在(T(A)\geq T\geq T(c))范围内从高温到低温动力学的转变。对单位密度的 Lennard-Jones 体系进行了分子动力学模拟，采用与相关局部最小值或固有结构（IS）的时间相关映射。引入的动力学量包括(r2(is)(t))，即 IS 吸引盆内的均方位移（MSD）；(R2(t))，即 IS 本身的 MSD；以及(g(t))，即 IS 等待时间的分布。将构型空间视为协同局部区域贡献的组合，并给出了一种从(g(t))获得物理上有意义的(g(loc)(t))和平均等待时间(\tau(loc))的方法。从(r2(is)(t))和(\tau(loc))的角度获得了对转变的理解。在中间温度(T)下，(r2(is)(t))具有线性(t)依赖区间，从而可以计算盆内扩散常数(D(is))。在接近(T(c))时，对于(t < \tau(loc))，盆内扩散已充分建立，扩散由盆内主导，(D = D(is))；(D)可以在一个盆内计算。在(T(c))以下，(\tau(loc))超过系统探索该盆所需的时间(\tau(pl))，这表明边界处存在障碍的作用；(\tau(loc)=\tau(pl))是向活化跳跃转变的一个判据。盆内扩散提供了一种不涉及障碍的限制方式，并且在(T(c))以上的动力学中起关键作用。讨论了(T(c))以下 IS 之间的运动（IS 动力学）与(T(c))以上鞍点或边界动力学之间的区别，在(T(c))以上，系统总是接近连接各盆的鞍点障碍之一，并且 IS 边界间距紧密且易于跨越。基于(R2(t))与传统 MSD 的关系引入了一个边界指数，并表明在(T\approx T(c))时该指数消失。有人提出盆内扩散是鞍点动力学的一种表现。
