冰（0001）表面分子重排过程的长时间模拟
Molecular reordering processes on ice (0001) surfaces from long timescale simulations
摘要：
我们报告了长时间尺度自适应动力学蒙特卡罗模拟的结果，旨在确定可能的分子重排过程的质子无序和有序（弗莱彻）基面（0001）表面的六边形冰。模拟基于柔性分子的力场，在100 K的温度下跨越长达50 μs的时间间隔，这代表了地球大气温度范围的下限。使用密度泛函理论和基于从头计算的极化势函数进行额外的计算，以测试和完善力场的预测。几个不同的过程被发现容易发生，即使在这种低的温度，包括协调的重新取向（翻转）的相邻表面分子，这改变了悬挂的H-原子的图案，并形成间隙缺陷的向下运动的上层双层分子。在质子无序表面上，观察到一个主要的表面粗糙化过程，其显著地破坏了晶体结构。尽管更长的模拟时间，这样的粗糙化过程中没有观察到的高度有序的弗莱彻表面，这是积极的更稳定，因为相邻的悬挂H-原子之间的排斥作用较小。然而，一个更本地化的过程发生在弗莱彻表面涉及的表面分子暂时离开其晶格位点。翻转过程提供了一个简单的途径，增加质子的顺序和稳定的表面，支持一个主要的弗莱彻样排序的低温冰表面，但我们的模拟也表明，质子无序补丁的表面上可能会引起显着的局部重建。此外，弗莱彻表面上的分子的子集易于形成间隙缺陷。
英文摘要：
We report results of long timescale adaptive kinetic Monte Carlo simulations aimed at identifying possible molecular reordering processes on both proton-disordered and ordered (Fletcher) basal plane (0001) surfaces of hexagonal ice. The simulations are based on a force field for flexible molecules and span a time interval of up to 50 μs at a temperature of 100 K, which represents a lower bound to the temperature range of Earth's atmosphere. Additional calculations using both density functional theory and an ab initio based polarizable potential function are performed to test and refine the force field predictions. Several distinct processes are found to occur readily even at this low temperature, including concerted reorientation (flipping) of neighboring surface molecules, which changes the pattern of dangling H-atoms, and the formation of interstitial defects by the downwards motion of upper-bilayer molecules. On the proton-disordered surface, one major surface roughening process is observed that significantly disrupts the crystalline structure. Despite much longer simulation time, such roughening processes are not observed on the highly ordered Fletcher surface which is energetically more stable because of smaller repulsive interaction between neighboring dangling H-atoms. However, a more localized process takes place on the Fletcher surface involving a surface molecule transiently leaving its lattice site. The flipping process provides a facile pathway of increasing proton-order and stabilizing the surface, supporting a predominantly Fletcher-like ordering of low-temperature ice surfaces, but our simulations also show that proton- disordered patches on the surface may induce significant local reconstructions. Further, a subset of the molecules on the Fletcher surface are susceptible to forming interstitial defects.
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