npj：莫一非——计算离子扩散的正确“姿势”
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第一性原理分子动力学（AIMD）模拟可以广泛用于研究材料的扩散机制和计算相应的材料扩散特性。然而，AIMD模拟通常局限在几百个原子的系统，且模拟时间也仅限于亚纳秒物理时域的范围，因此仅能模拟到有限数量的随机扩散事件。这致使从AIMD模拟中得到的扩散结果往往受到统计误差的影响。本研究重新审视了通过AIMD模拟估算扩散系数和离子电导率的过程，并建立了具有最小拟合误差程序。此外，我们提出了相应的方法，能通过AIMD模拟到的扩散事件数量来量化扩散系数和离子电导率的统计方差。由于扩散事件数量的采样必须达到足够的数目，因此AIMD模拟时间应足够长，而且只能在具有快速扩散的材料上进行研究。我们界定了应用AIMD模拟研究扩散特性所适用的材料范围和物理条件。本研究量化了从AIMD模拟得到的扩散结果的统计置信度，以及为正确应用这一强大技术奠定了基础。该文近期发表于npj Computational Materials 4: 18 (2018); doi: 10.1038/s41524-018-0074-y。英文标题与摘要如下，点击阅读原文可以自由获取论文PDF。
Statistical variances of diffusional properties from ab initio molecular dynamics simulations
Xingfeng He, Yizhou Zhu,
Alexander Epstein & Yifei Mo
Ab initio molecular dynamics (AIMD) simulation is widely employed in studying diffusion mechanisms and in quantifying diffusional properties of materials. However, AIMD simulations are often limited to a few hundred atoms and a short, sub-nanosecond physical timescale, which leads to models that include only a limited number of diffusion events. As a result, the diffusional properties obtained from AIMD simulations are often plagued by poor statistics. In this paper, we re-examine the process to estimate diffusivity and ionic conductivity from the AIMD simulations and establish the procedure to minimize the fitting errors. In addition, we propose methods for quantifying the statistical variance of the diffusivity and ionic conductivity from the number of diffusion events observed during the AIMD simulation. Since an adequate number of diffusion events must be sampled, AIMD simulations should be sufficiently long and can only be performed on materials with reasonably fast diffusion. We chart the ranges of materials and physical conditions that can be accessible by AIMD simulations in studying diffusional properties. Our work provides the foundation for quantifying the statistical confidence levels of diffusion results from AIMD simulations and for correctly employing this powerful technique.
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