Malek Shahrazad M A, Morrow Gregory P, Saika-Voivod Ivan
Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John's, Newfoundland and Labrador A1B 3X7, Canada.
J Chem Phys. 2015 Mar 28;142(12):124506. doi: 10.1063/1.4915917.
We carry out molecular dynamics (MD) and Monte Carlo (MC) simulations to characterize nucleation in liquid clusters of 600 Lennard-Jones particles over a broad range of temperatures. We use the formalism of mean first-passage times to determine the rate and find that Classical Nucleation Theory (CNT) predicts the rate quite well, even when employing simple modelling of crystallite shape, chemical potential, surface tension, and particle attachment rate, down to the temperature where the droplet loses metastability and crystallization proceeds through growth-limited nucleation in an unequilibrated liquid. Below this crossover temperature, the nucleation rate is still predicted when MC simulations are used to directly calculate quantities required by CNT. Discrepancy in critical embryo sizes obtained from MD and MC arises when twinned structures with five-fold symmetry provide a competing free energy pathway out of the critical region. We find that crystallization begins with hcp-fcc stacked precritical nuclei and differentiation to various end structures occurs when these embryos become critical. We confirm that using the largest embryo in the system as a reaction coordinate is useful in determining the onset of growth-limited nucleation and show that it gives the same free energy barriers as the full cluster size distribution once the proper reference state is identified. We find that the bulk melting temperature controls the rate, even though the solid-liquid coexistence temperature for the droplet is significantly lower. The value of surface tension that renders close agreement between CNT and direct rate determination is significantly lower than what is expected for the bulk system.
我们进行了分子动力学（MD）和蒙特卡罗（MC）模拟，以表征600个 Lennard-Jones 粒子的液体团簇在很宽温度范围内的成核过程。我们使用平均首次通过时间的形式来确定速率，发现经典成核理论（CNT）能很好地预测速率，即使采用微晶形状、化学势、表面张力和粒子附着速率的简单模型，直至液滴失去亚稳性且结晶通过非平衡液体中生长受限的成核进行的温度。在这个交叉温度以下，当使用MC模拟直接计算CNT所需的量时，仍然可以预测成核速率。当具有五重对称性的孪晶结构提供了一条从临界区域出来的竞争自由能路径时，从MD和MC获得的临界胚胎尺寸会出现差异。我们发现结晶始于hcp - fcc堆叠的预临界核，当这些胚胎变得临界时会分化为各种最终结构。我们证实，将系统中最大的胚胎用作反应坐标有助于确定生长受限成核的起始，并表明一旦确定了适当的参考状态，它给出的自由能垒与完整团簇尺寸分布相同。我们发现，尽管液滴的固液共存温度明显更低，但体相熔化温度控制着速率。使CNT与直接速率测定结果密切一致的表面张力值明显低于体相系统的预期值。
