Order-parameter-aided temperature-accelerated sampling for the exploration of crystal polymorphism and solid-liquid phase transitions.
Order-parameter-aided temperature-accelerated sampling for the exploration of crystal polymorphism and solid-liquid phase transitions.
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DOI：
10.1063/1.4878665
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发表时间：
2014-06
期刊：
影响因子：
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通讯作者：
Tang-Qing Yu;Pei-Yang Chen;Ming Chen;A. Samanta;E. Vanden-Eijnden;M. Tuckerman
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文献类型：
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作者：
Tang-Qing Yu;Pei-Yang Chen;Ming Chen;A. Samanta;E. Vanden-Eijnden;M. Tuckerman
预测原子和分子晶体的多态性问题在实验和理论上都构成了重大挑战。从理论角度来看，多态性预测属于以潜在的粗糙能量景观为特征的一般问题，因此，基于自由能的增强采样方法可以解决该问题。在本文中，我们建立在两位作者先前介绍的方案的基础上，其中超级单元的长度和角度是通过温度加速绝热自由能动力学来增强采样的目标[T. Q. Yu 和 M. E. Tuckerman，物理学家。莱特牧师。 107, 015701 (2011)]。在这里，该框架被扩展为包括区分不同晶体排列的一般有序参数，作为增强采样的目标集体变量。由此产生的自由能表面具有相当高的维度，重建起来并不容易，我们讨论了一种执行自由能分析的特定策略。该方法适用于使用 Steinhardt 阶参数研究高压和高温下氙晶体的多态性，而超晶胞不包含在集体变量集中。获得了预期的 fcc 和 bcc 结构，并且当将超晶胞参数作为集体变量包含在内时，我们还发现了几个新结构，包括具有 hcp 堆垛层错的 fcc 状态。我们还使用相同的 Steinhardt 阶参数将新方法应用于 1300 K 下铜的固液相变。我们的方法能够反复熔化和重新冻结系统，并且可以高效地获得自由能剖面。
The problem of predicting polymorphism in atomic and molecular crystals constitutes a significant challenge both experimentally and theoretically. From the theoretical viewpoint, polymorphism prediction falls into the general class of problems characterized by an underlying rough energy landscape, and consequently, free energy based enhanced sampling approaches can be brought to bear on the problem. In this paper, we build on a scheme previously introduced by two of the authors in which the lengths and angles of the supercell are targeted for enhanced sampling via temperature accelerated adiabatic free energy dynamics [T. Q. Yu and M. E. Tuckerman, Phys. Rev. Lett. 107, 015701 (2011)]. Here, that framework is expanded to include general order parameters that distinguish different crystalline arrangements as target collective variables for enhanced sampling. The resulting free energy surface, being of quite high dimension, is nontrivial to reconstruct, and we discuss one particular strategy for performing the free energy analysis. The method is applied to the study of polymorphism in xenon crystals at high pressure and temperature using the Steinhardt order parameters without and with the supercell included in the set of collective variables. The expected fcc and bcc structures are obtained, and when the supercell parameters are included as collective variables, we also find several new structures, including fcc states with hcp stacking faults. We also apply the new method to the solid-liquid phase transition in copper at 1300 K using the same Steinhardt order parameters. Our method is able to melt and refreeze the system repeatedly, and the free energy profile can be obtained with high efficiency.