Liquid structure simulation studies

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. 

It is now well-established that for atomic fluids, far from the critical point, the atomic organisation is dictated by the repulsive forces while the longer range attractive forces serve to maintain the high density [34]. The investigation of systems of hard spheres can therefore be used as simple models for atomic systems they also serve as a basis for a thermodynamic perturbation analysis to introduce the attractive forces in a van der Waals-like approach [35]. In consequence it is to be expected that the anisotropic repulsive forces would be responsible for the structure of liquid crystal phases and numerous simulation studies of hard objects have been undertaken to explore this possibility [36]. 

Cabral BJC, Rivail JL, Bigot B (1987) A Monte-Carlo simulation study of a polarizable liquid -influence of the electrostatic induction on its thermodynamic and structural-properties. J Chem Phys... 

This chapter is concerned with the application of liquid state methods to the behavior of polymers at surfaces. The focus is on computer simulation and liquid state theories for the structure of continuous-space or off-lattice models of polymers near surfaces. The first computer simulations of off-lattice models of polymers at surfaces appeared in the late 1980s, and the first theory was reported in 1991. Since then there have been many theoretical and simulation studies on a number of polymer models using a variety of techniques. This chapter does not address or discuss the considerable body of literature on the adsorption of a single chain to a surface, the scaling behavior of polymers confined to narrow spaces, or self-consistent field theories and simulations of lattice models of polymers. The interested reader is instead guided to review articles [9-11] and books [12-15] that cover these topics. 

The OPLS parameters (charges and Lennard-Jones terms) were obtained primarily via Monte Carlo simulations with particular emphasis on reproducing the experimental densities and heats of vaporization of liquids. Those simulations were performed iteratively as part of the parametrization, so better agreement with experiment is obtained than in previous studies where the simulations were usually carried out after the parametrization. Once the OPLS parametrization was completed, further simulations were also performed in order to test the new set of parameters in the calculation of other thermodynamic and structural properties of the system, besides its density and its heat of vaporization. Parameters have now been generated, among others, for water, alkanes, alkenes, alcohols, amides, alkyl chlorides, amines, carboxylic esters and acids, various sulfur and nitrogen compounds, and nitriles. A protein force field has been established as well. 

One of the further refinements which seems desirable is to modify Eq. (9) so that it has wiggles (damped oscillatory behavior). Wiggles are expected in any realistic MM-level pair potential as a consequence of the molecular structure of the solvent (2,3,10,11,21,22) they would be found even for two hard sphere solute particles in a hard-sphere liquid or for two H2I80 solute molecules in ordinary liquid HpO, and are found in simulation studies of solutions based on BO-level models. In ionic solutions in a polar solvent another source of wiggles, evidenced in Fig. 2, may be associated with an oscillatory nonlocal dielectric function e(r). ( 36) These various studies may be used to guide the introduction of wiggles into Eq. (9) in a realistic way. 

Zhang, L., Sun, L., Siepmann, J.I., and Schure, M.R., Molecular simulation study of the bonded-phase structure in reversed-phase liquid chromatography with neat aqueous solvent, J. Chmmatogr. A, 1079, 127, 2005. 

Urahata, S.M., and Ribeiro, M.C.C., Structure of ionic liquids of l-alkyl-3-methy-limidazolium cations A systematic computer simulation study, ]. Chem. Phys., 120, 1855-1863, 2004. 

Bhargava, B.L., and Balasubramanian, S., Intermolecular structure and dynamics in an ionic liquid A Car-Parrinello molecular dynamics simulation study of 1,3-dimethylimidazolium chloride, Chem. Phys. Lett., 417, 486-491, 2006. 

Shah, J. K., and Maginn, E. J., A Monte Carlo simulation study of the ionic liquid l-n-butyl-3-methylimidazolium hexafluorophosphate Liquid structure, volumetric properties and infinite dilution solution thermodynamics of CO2, Fluid Phase Equilb., 222-223, 195-203, 2004. 

Peter Kusalik took up an appointment at Dalhousie University in 1989 and developed a research program focused on computer simulation studies of molecular liquids, solids, and solutions. As well as standard simulation approaches, he has explored nonequilibrium molecular dynamics techniques and applied field simulations, the development of new models and methodologies being one aim of his research. A common focus throughout his work has been the examination of the interplay between microscopic structure and dynamics in condensed matter and their relationship to bulk properties. 

B. M. Ladanyi, Computer simulation studies of solvation dynamics in mixtures, in J. Samios and V. A. Durov (eds), Novel Approaches to the Structure and Dynamics of Liquids Experiments, Theories and Simulations, Kluwer, Dordrecht, 2004, NATO Sci. Ser. II, Vol. 133, p. 560. 

Xenides D, Randolf BR et al (2005) Structure and ultrafast dynamics of liquid water a quantum mechanics/ molecular mechanics molecular dynamics simulations study. J Chem Phys 122 4506... 

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