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Thermostatting Open Systems in Non-Equilibrium Computer Simulations

非平衡计算机模拟中的恒温开放系统

基本信息

批准号：
EP/J019259/1
负责人：
Lev Kantorovitch
金额：
$ 49.65万
依托单位：
King's College London
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2013
资助国家：
英国
起止时间：
2013 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FJ019259%2F1
关键词：
Thermostatting Open Systems Non Equilibrium

项目摘要

Molecular Dynamics (MD) simulations, in which atoms move according to Newtonian's dynamics, have been extensively used to study various processes in matter in which materials are under considerable mechanical stress, subjected to a temperature gradient, irradiated upon by high-energy particles, etc. For instance, in tribology applications, two surfaces are sheared with respect to each other, and bonds are constantly formed and broken in the contact area leading to friction; this results in the contact area being much hotter than the bulk of the two materials leading to constant energy flow from the contact region outwards into the bulk. Correct treatment of the effects responsible for describing energy dissipation from the hot region into the bulk of the materials is crucial for a realistic description of friction. As another example, in irradiation processes high-energy particles impinged on a crystal surface (e.g. in coatings of nuclear reactors) go through the material forming canals. Locally along the canal large amounts of energy are released leading to considerable damage (deformation, defects formation, etc.); at the same time, a large portion of that energy is dissipated inside the material propagating the damage radially out of the canal. Correct treatment of such dissipation effects is vital for simulating the realistic damage to the material and hence finding new materials, which can sustain higher radiation doses. In these and many other cases, due to considerable computational cost, only a small fragment of the material can actually be studied at the atomic level by practical MD simulations. At the same time, violent processes releasing considerable amounts of energy, which dissipates into the bulk of the material(s), require considering essentially infinite systems. Thermostatting is meant to solve this problem by providing mechanisms whereby a finite fragment of the real system is simulated, however, the environment is mimicked as a heat bath (kept at constant temperature), which can take or give energy in accordance with the laws of thermodynamics. Unfortunately, in very many cases, equilibrium MD simulations are still frequently used although this can hardly be justified! This still happens because there is no viable alternative. The well-known Generalised Langevin Equation (GLE) method is specifically designed to take care of the energy exchange with the environment (the heat bath), in spite of its long history, GLE has not yet been exploited sufficiently and implemented into a code to offer a practical solution. The know-how generated by this research project offers this solution. The methodology to be generated and the computer codes (both the full implementation of GLE and the approximate schemes) will be applicable to simulations of a wide class of non-equilibrium phenomena such as (but not limited to): tribology; thermal transport through bulk materials and layered systems, nanowires and molecular junctions; relaxation of point defects in the bulk and surfaces of crystals; irradiation problems; film and crystals growth on substrates at elevated temperatures (e.g. epitaxial), etc.

分子动力学（MD）模拟，其中原子根据牛顿动力学运动，已被广泛用于研究物质中的各种过程，其中材料处于相当大的机械应力下，经受温度梯度，被高能粒子照射等。例如，在摩擦学应用中，两个表面相对于彼此剪切，并且在接触区域中不断地形成和破坏键，导致摩擦;这导致接触区域比两种材料的本体热得多，从而导致从接触区域向外进入本体的恒定能量流。正确处理的影响，负责描述能量耗散从热区到大部分的材料是至关重要的一个现实的描述摩擦。作为另一个例子，在辐照过程中，撞击在晶体表面（例如，在核反应堆的涂层中）上的高能粒子穿过形成通道的材料。局部地沿着管道释放大量能量，导致相当大的损坏（变形、缺陷形成等）;与此同时，大部分能量在材料内部耗散，从而将损伤径向地传播到管道之外。正确处理这种耗散效应对于模拟材料的真实损伤并因此找到能够承受更高辐射剂量的新材料至关重要。在这些和许多其他情况下，由于相当大的计算成本，实际上只有一小部分材料可以通过实际的MD模拟在原子水平上进行研究。与此同时，释放大量能量的剧烈过程，这些能量消散在大量的材料中，需要考虑基本上无限的系统。恒温是为了解决这个问题，通过提供机制，其中一个有限的片段的真实的系统被模拟，然而，环境被模仿为一个热浴（保持在恒定的温度），它可以采取或给予能量根据热力学定律。不幸的是，在很多情况下，平衡MD模拟仍然经常使用，尽管这几乎是不合理的！这种情况仍然存在，因为没有可行的替代方案。众所周知的广义朗之万方程（GLE）方法是专门设计来处理与环境（热浴）的能量交换的，尽管其历史悠久，但GLE尚未被充分利用并实施到代码中以提供实用的解决方案。该研究项目产生的专有技术提供了这种解决方案。拟采用的方法和计算机代码（GLE的完全实现和近似方案）将适用于模拟广泛的一类非平衡现象，（但不限于）：摩擦学;通过大块材料和层状系统、纳米线和分子结的热传输;晶体块体和表面中的点缺陷的弛豫;辐照问题;在升高的温度下在衬底上的膜和晶体生长（例如外延生长）等。