Molecular simulation Subject

The molecular and liquid properties of water have been subjects of intensive research in the field of molecular science. Most theoretical approaches, including molecular simulation and integral equation methods, have relied on the effective potential, which was determined empirically or semiempirically with the aid of ab initio MO calculations for isolated molecules. The potential parameters so determined from the ab initio MO in vacuum should have been readjusted so as to reproduce experimental observables in solutions. An obvious problem in such a way of determining molecular parameters is that it requires the reevaluation of the parameters whenever the thermodynamic conditions such as temperature and pressure are changed, because the effective potentials are state properties. 

One of the most important new areas of theory of charge transfer reactions is direct molecular simulations, which allows for an unprecedented, molecular level view of solvent motion during reactions in this class. One of the important themes for research of this type is to ascertain the validity at a molecular level of the linear response theory estimates of solvent interactions that are inherent in Marcus theory and related approaches. In addition, the importance of dynamic solvent effects on charge transfer kinetics is being examined. Recent papers on this subject have been published by Warshel [71], Hynes [141] and Bader and Chandler [137, 138],... 

Brief descriptions of fundamental techniques such as quantum mechanical calculations, molecular mechanics, molecular dynamics, and Monte Carlo methods are given, with a particular emphasis on aspects that make the molecular simulation of polymers and of low molecular weight liquids different. Two very good books on the latter subject are those by Allen and Tildesley and by Hansen and MacDonald. ... 

Harrison et al. reported the first w/c microemulsion in 1994 (20). A hybrid surfactant, namely F7H7, made of respectively one hydrocarbon and one fluorocarbon chain attached onto the same sulfate head group, was able to stabilize a w/c microemulsion at 35 and 262 bar. For a surfactant concentration of 1.9 wt %, water up to a w = 32 value ([water]/surfactant]) could be dispersed. A spherical micellar structure was confirmed by small-angle neutron scattering (SANS) experiments (21). This surfactant was later the subject of dynamic molecular simulations (22, 23). The calculations were consistent with the SANS data and high diffusivity was predicted, highlighting this important feature of low-density and low-viscosity supercritical fluids (SCF). 

The basic idea behind an atomistic-level simulation is quite simple. Given an accurate description of the energetic interactions between a collection of atoms and a set of initial atomic coordinates (and in some cases, velocities), the positions (velocities) of these atoms are advanced subject to a set of thermodynamic constraints. If the positions are advanced stochastically, we call the simulation method Monte Carlo or MG [10]. No velocities are required for this technique. If the positions and velocities are advanced deterministically, we call the method molecular dynamics or MD [10]. Other methods exist which are part stochastic and part deterministic, but we need not concern ourselves with these details here. The important point is that statistical mechanics teUs us that the collection of atomic positions that are obtained from such a simulation, subject to certain conditions, is enough to enable aU of the thermophysical properties of the system to be determined. If the velocities are also available (as in an MD simulation), then time-dependent properties may also be computed. If done properly, the numerical method that generates the trajectories... 

As noted, the field of molecular simulation is relatively new, and a detailed review of it is beyond the scope of this text and we introduce here a few of the more relevant references. One of the first applications of molecular mechanics to polymers was by Theodorou and Suter (93,94), who modeled atactic polypropylene as an amorphous cell subjected to a range of stress conditions (hydrostatic pressure, pure strain, and uniaxial strain). Such modeling generally gives reasonable estimates of the elastic constants of a material [within 15% (79)], providing the density of the glass is correctly modeled. 

One of the main reasons for using computer simulations is the fact that they eliminate inaccuracies resulting from the approximate statistical thermodynamic methods. In terms of computer simulations methods, it is possible to investigate the systems not subjected to the analytical description. As the methods under investigations can be used to study the complex systems, they provide the standard data for verification of approximate theories. Moreover, they make it possible to compare the molecular models with the experimental data as well as to form the correctness criteria for their choice. Thus, the molecular simulation methods are useful for testing the exactness of assumed intermolecular potentials, to validate approximate statistical thermodynamical theories and to explore systems under conditions and with a level of detail which is hardly to achieve by conventional experiment. 

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