通过增强离子缺陷形成以分子动力学模拟研究冰Ih/XI相变
Enhancing the formation of ionic defects to study the ice Ih/XI transition with molecular dynamics simulations
AI总结：
本研究提出在分子动力学模拟中用偏置势增强离子缺陷形成以模拟KOH掺杂效应，结合可模拟分子解离的机器学习势，加速质子构型扩散，研究冰Ih/XI有序-无序相变，计算得到的相变熵在误差范围内与实验一致。
摘要：
冰Ih是生物圈中常见的冰的形态，具有质子无序特性。其质子有序的对应物冰XI在72 K以下具有热力学稳定性。然而，即便低于该温度，冰XI的形成也受动力学阻碍，实验中需通过KOH掺杂冰来获得。掺杂会产生离子缺陷，促进质子迁移以及相关的质子构型变化。本文中，我们在分子动力学模拟中利用偏置势增强离子缺陷的形成，以此模拟掺杂效应。由此形成的离子通过氢氧根的快速迁移发生复合，并导致质子沿氢键环跃迁。这为改变质子构型、加速质子构型空间中的扩散提供了一种物理且高效的途径。该方法的关键要素是一个基于密度泛函理论数据训练的机器学习势，能够模拟分子解离。我们通过使用合适的序参量区分冰Ih和冰XI中的质子环境，研究有序-无序相变，验证了这一思路的有效性。我们计算了与该相变相关的自由能、焓和熵的变化。我们估算的熵值在计算误差范围内与实验结果一致。
英文摘要：
Ice Ih, the common form of ice in the biosphere, contains proton disorder. Its proton-ordered counterpart, ice XI, is thermodynamically stable below 72 K. However, even below this temperature the formation of ice XI is kinetically hindered and experimentally it is obtained by doping ice with KOH. Doping creates ionic defects that promote the migration of protons and the associated change in proton configuration. In this article, we mimic the effect of doping in molecular dynamics simulations using a bias potential that enhances the formation of ionic defects. The recombination of the ions thus formed proceeds through fast migration of the hydroxide and results in the jump of protons along a hydrogen bond loop. This provides a physical and expedite way to change the proton configuration, and to accelerate diffusion in proton configuration space. A key ingredient of this approach is a machine learning potential trained with density functional theory data and capable of modeling molecular dissociation. We exemplify the usefulness of this idea by studying the order-disorder transition using an appropriate order parameter to distinguish the proton environments in ice Ih and XI. We calculate the changes in free energy, enthalpy, and entropy associated with the transition. Our estimated entropy agrees with experiment within the error bars of our calculation.
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