Surfaces atomistic structures

A. J. ROSENBERG (M. I. T.) r Throughout your paper you treat the surface as a physical continuum, disregarding its atomistic structure. Chemically we know that we must consider the atomistic nature of the surface I wonder if you might suggest how this aspect of the surface might be incorporated in your treatment. 

The atomistic structure and dynamics of the interaction of an atomic force microscopic probe (AFM) with a crystalline polyethylene surface was examined by using the molecular dynamics method coupled with ab initio quantum calculations [21]. A set of force parameters and guidelines have been derived from the extensive computer simulations, and these results were used to help explain some of the AFM images. In general, AFM experiments can be performed in a nondestructive mode with a reasonable resolution, provided that the forces of interaction between a typical-size tip and sample are kept within the... 

The AIMD models of the surface region represent a necessary starting point for any large-scale MD study of the interface. The limited information available on the atomistic structure of the glass substrate complicates the assessment of the accuracy of surface models obtained by classical force fields. The AIMD models, albeit of limited size, are an essential reference to define the stability of typical surface sites, that should be at least qualitatively reflected in any larger model produced by classical MD simulations. Whereas the latter are generally not suitable to investigate surface... 

B. Folleher and K. E. Heitsler, The mechanism of the iron electrode and the atomistic structure of iron surfaces, J. Electroanal. Chem. 180 11 (1984). 

The atomistic structure of a surface is not generally a simple termination of the bulk material the surface atoms undergo relaxation and rumpling. This is hardly surprising as the environment, such as the coordination number, of a surface atom is profoundly different from that of an equivalent atom in the bulk. The surface relaxation can be captured using energy minimisation or molecular dynamics, which direct the (surface) atoms into low-energy... 

Figure 5.4 Atomistic structure of a screw-edge dislocation in MgO. The atoms comprising the core, which protrude out of the surface, are shown as dark (oxygen) and light (magnesium) spheres. Reproduced from Sayle with permission from the Royal Society of Chemistry.

Figure 5.11 Atomistic structures of ceria nanoparticles, (a)-(d) Snapshots taken during an MD simuladon showing the crystallisation of a Ti-CeO nanoparticle from a molten precursor, (e) Model atomistic structure of a Ce02 nanoparticle as it starts to solidify the oval indicates the nucleating seed, which spontaneously evolves on the surface, (f) Surface rendered model of a Ce02 nanoparticle after...

Fabris S, Vicario G, Balducci G, de Gironcoli S, Baroni S Electronic and atomistic structures of clean and reduced ceria surfaces, J Phys Chem B 109(48) 22860—22867, 2005. 

Hence, it is imperative that we understand the surface atomistic structure of the glass. Molecular modeling has been widely used in developing an understanding of the atomic structure of glasses and their surfaces and has proved a powerful tool for numerous systems [67-73]. In the example presented in this section, molecular dynamics simulations are used to build surfaces of sodium silicate glass and... 

In this situation computer simulation is useful, since the conditions of the simulation can be chosen such that full equihbrium is established, and one can test the theoretical concepts more stringently than by experiment. Also, it is possible to deal with ideal and perfectly flat surfaces, very suitable for testing the general mechanisms alluded to above, and to disregard in a first step all the complications that real substrate surfaces have (corrugation on the atomistic scale, roughness on the mesoscopic scale, surface steps, adsorbed impurities, etc.). Of course, it may be desirable to add such complications at a later stage, but this will not be considered here. In fact, computer simulations, i.e., molecular dynamics (MD) and Monte Carlo (MC) calculations, have been extensively used to study both static and dynamic properties [11] in particular, structural properties at interfaces have been considered in detail [12]. 

This article reviews progress in the field of atomistic simulation of liquid crystal systems. The first part of the article provides an introduction to molecular force fields and the main simulation methods commonly used for liquid crystal systems molecular mechanics, Monte Carlo and molecular dynamics. The usefulness of these three techniques is highlighted and some of the problems associated with the use of these methods for modelling liquid crystals are discussed. The main section of the article reviews some of the recent science that has arisen out of the use of these modelling techniques. The importance of the nematic mean field and its influence on molecular structure is discussed. The preferred ordering of liquid crystal molecules at surfaces is examined, along with the results from simulation studies of bilayers and bulk liquid crystal phases. The article also discusses some of the limitations of current work and points to likely developments over the next few years. 

The rapid rise in computer speed over recent years has led to atom-based simulations of liquid crystals becoming an important new area of research. Molecular mechanics and Monte Carlo studies of isolated liquid crystal molecules are now routine. However, care must be taken to model properly the influence of a nematic mean field if information about molecular structure in a mesophase is required. The current state-of-the-art consists of studies of (in the order of) 100 molecules in the bulk, in contact with a surface, or in a bilayer in contact with a solvent. Current simulation times can extend to around 10 ns and are sufficient to observe the growth of mesophases from an isotropic liquid. The results from a number of studies look very promising, and a wealth of structural and dynamic data now exists for bulk phases, monolayers and bilayers. Continued development of force fields for liquid crystals will be particularly important in the next few years, and particular emphasis must be placed on the development of all-atom force fields that are able to reproduce liquid phase densities for small molecules. Without these it will be difficult to obtain accurate phase transition temperatures. It will also be necessary to extend atomistic models to several thousand molecules to remove major system size effects which are present in all current work. This will be greatly facilitated by modern parallel simulation methods that allow molecular dynamics simulations to be carried out in parallel on multi-processor systems [115]. 

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