Thermostatting Open Systems in Non-Equilibrium Computer Simulations

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.

This research is internationally leading and will be published in high impact journals (Nature Group Journals, Science, PRL, etc.). LK and CL have delivered over 40 invited talks at international conferences. This scientific exposure will ensure international impact and increase the profile of UK science.
Results of the research relevant to industries will be disseminated with the help of KCL Business and the Thomas Yong Centre (TYC). LK is on the TYC Executive Committee (TYC EC) and participates in its decision-making. Currently, TYC has close contacts with National Physics Laboratory, which has extensive industry contacts; recently, direct contacts were established with Samsung, UK Defence Industry and BP, and the work in finding more industry contacts is ongoing. LK and CL, as well as the PDRAs employed on the project, will scrutinise the outputs of the research for possible exploitation, aided by KCL Business.
A specific industrial interest in the results of our research is anticipated in the following areas: (i) growth of thin atomic and molecular layers on surfaces at elevated temperatures for applications in coatings, conducting layers, sensors, e.g. epitaxial growth of graphene on transition metal surfaces; (ii) shielding materials for nuclear reactors: finding new materials requires simulating their damage by high energy particles impinging on them; (iii) thermal conductivity of materials and structures, e.g. for nanotechnology applications: reducing thermal losses in thermoelectric materials for applications e.g. in electric power generation and solid-state cooling, is pivotal for increasing their performance (determined by the dimensionless thermoelectric figure of merit (ZT) inversely proportional to the effective thermal conductivity); (iv) energy materials, e.g. fuel cells: understanding chemical processes involved requires realistic MD simulations to study reactions at the electrodes where energy is consumed/released; (v) mechanical resistivity of materials against deformation and rupture requires applying proper thermostatting to dissipate released energy; (vi) pharmaceutical applications of metallic nanoparticles, which can be heated with light to potentially release drugs from polymer capsules or destroy malignant cancer cells. The development of theoretical models and tools will transform the predictive power of MD simulations under realistic conditions, benefiting industry. In turn, cheaper products will be available with better properties and consuming less energy, and this should benefit the society at large by improving the quality of life.
Public Engagement and Outreach. Results of wider significance to technology and society will be disseminated with the help of a dedicated science media liaison person, employed recently by the School at KCL. She will communicate our outcomes to wider mainstream media greatly expanding the targeted audience. Also, the output of the research will be disseminated at weekly Maxwell lectures, hosted by KCL, which are free to attend for the general public. Physics Department has an active outreach committee, which will be constantly updated with new initiatives and lectures resulting from the results of our research, and activities involving several London based secondary schools, and the results of our research will be made available to the students of these schools to foster their interest in physics via seminars and invited lectures; at the same time, teachers would have an opportunity to learn more about the science we have been doing. The PI has already been invited by David Smith from Highgate School to give a presentation about his research. Other schools, especially from deprived areas, will be specifically targeted. These activities should serve to increase the public understanding and interest in science.