Understanding high pressure molecular hydrogen with a hierarchical machine-learned potential.
Understanding high pressure molecular hydrogen with a hierarchical machine-learned potential.
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DOI：
10.1038/s41467-020-18788-9
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发表时间：
2020-10-06
影响因子：
16.6
通讯作者：
Ackland GJ
中科院分区：
文献类型：
--
作者：
Zong H;Wiebe H;Ackland GJ
氢相图有几个不同寻常的特征，密度泛函计算很好地再现了这些特征。不幸的是，这些计算并没有提供很好的物理洞察力来解释为什么会出现这些特征。在这里，我们提出了一种快速的原子间相互作用势，它再现了分子氢相：取向无序的第一相、破坏对称的第二相和重入熔化曲线。氢原子的振动频率在高压下下降是因为相邻分子之间的耦合增强，而不是键的减弱。在高压下，液态氢的密度比并存的紧密堆积固体高，这是因为受青睐的分子取向从四极极能量最小化转变为空间斥力最小化。后者允许分子更紧密地聚集在一起，而不会使原子更紧密，但由于受挫，无法在紧密堆积的层中实现。类似的效应会导致负的热膨胀。在高压下，相I的旋转受到阻碍，因此不能将其视为分子转子相。氢具有多个分子相，这在计算上的探索是具有挑战性的。作者开发了一种机器学习方法，从参考从头算分子动力学模拟中学习，以推导出一个可转移的层次力模型，该模型提供了对高压相和氢的熔融线的洞察。
The hydrogen phase diagram has several unusual features which are well reproduced by density functional calculations. Unfortunately, these calculations do not provide good physical insights into why those features occur. Here, we present a fast interatomic potential, which reproduces the molecular hydrogen phases: orientationally disordered Phase I; broken-symmetry Phase II and reentrant melt curve. The H2 vibrational frequency drops at high pressure because of increased coupling between neighbouring molecules, not bond weakening. Liquid H2 is denser than coexisting close-packed solid at high pressure because the favored molecular orientation switches from quadrupole-energy-minimizing to steric-repulsion-minimizing. The latter allows molecules to get closer together, without the atoms getting closer, but cannot be achieved within in a close-packed layer due to frustration. A similar effect causes negative thermal expansion. At high pressure, rotation is hindered in Phase I, such that it cannot be regarded as a molecular rotor phase. Hydrogen has multiple molecular phases which are challenging to explore computationally. The authors develop a machine-learning approach, learning from reference ab initio molecular dynamics simulations, to derive a transferable hierarchical force model that provides insight into high pressure phases and the melting line of H2.
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