van Meel J A, Page A J, Sear R P, Frenkel D
FOM Institute for Atomic and Molecular Physics, Amsterdam, The Netherlands.
J Chem Phys. 2008 Nov 28;129(20):204505. doi: 10.1063/1.3026364.
We present the results of Monte Carlo simulations of crystal nucleation from the vapor phase. We studied the Lennard-Jones system at conditions close to, but below, the triple point. This system is expected to show surface melting. The nucleation pathway that we observe consists of two distinct steps. In the first step, a liquid droplet nucleates from the vapor. Its nucleation rate depends strongly on the vapor supersaturation. In the second step, the final crystal phase nucleates in the liquid droplet, provided that this liquid droplet exceeds a minimum size. Our simulations show that within a liquid droplet the crystal nucleation rate does not depend on the vapor supersaturation. In a recent independent study Chen et al. [J. Phys. Chem. B 112, 4069 (2008)] investigated the same phenomenon using umbrella sampling to compute free energy barriers and hence nucleation rates. We use a different numerical approach where we focus on computing the nucleation rates directly using forward-flux sampling. Our results agree with the findings of Chen et al. and both methods observe two-step nucleation. This finding indicates that this nucleation process can be described with a quasiequilibrium theory. Due to different cutoffs for the interaction potential the results cannot be compared quantitatively.
我们展示了从气相中进行晶体成核的蒙特卡罗模拟结果。我们研究了处于接近但低于三相点条件下的 Lennard-Jones 系统。该系统预计会出现表面熔化现象。我们观察到的成核途径由两个不同的步骤组成。第一步，从气相中形成一个液滴。其成核速率强烈依赖于气相过饱和度。第二步，只要这个液滴超过最小尺寸，最终的晶相就在液滴中形成。我们的模拟表明，在液滴内部，晶体成核速率不依赖于气相过饱和度。在最近的一项独立研究中，Chen 等人 [《物理化学杂志 B》112, 4069 (2008)] 使用伞形抽样来计算自由能垒从而计算成核速率，对同一现象进行了研究。我们使用了一种不同的数值方法，即专注于直接使用前向通量抽样来计算成核速率。我们的结果与 Chen 等人的发现一致，两种方法都观察到了两步成核现象。这一发现表明，这个成核过程可以用准平衡理论来描述。由于相互作用势的截断值不同，结果无法进行定量比较。
