Simulating Liquids

This chapter focuses on the simulation of bulk liquids. This is a dilferent task from modeling solvation effects, which are discussed in Chapter 24. Solvation effects are changes in the properties of the solute due to the presence of a solvent. They are defined for an individual molecule or pair of molecules. This chapter discusses the modeling of bulk liquids, which implies properties that are not defined for an individual molecule, such as viscosity. 

The simplest case of fluid modeling is the technique known as computational fluid dynamics. These calculations model the fluid as a continuum that has various properties of viscosity, Reynolds number, and so on. The flow of that fluid is then modeled by using numerical techniques, such as a finite element calculation, to determine the properties of the system as predicted by the Navier-Stokes equation. These techniques are generally the realm of the engineering community and will not be discussed further here. 

Nearly all liquid simulations have been done using molecular mechanics force fields to describe the interactions between molecules. A few rare simulations have been completed with orbital-based methods. It is expected that it will still be a long time before orbital-based simulations represent a majority of the studies done due to the incredibly large amount of computational resources necessary for these methods. 

Monte Carlo simulations are an efficient way of predicting liquid structure, including the preferred orientation of liquid molecules near a surface. This is an efficient method because it is not necessary to compute energy derivatives, thus reducing the time required for each iteration. The statistical nature of these simulations ensures that both enthalpic and entropic effects are included. 

Molecular dynamics calculations are more time-consuming than Monte Carlo calculations. This is because energy derivatives must be computed and used to solve the equations of motion. Molecular dynamics simulations are capable of yielding all the same properties as are obtained from Monte Carlo calculations. The advantage of molecular dynamics is that it is capable of modeling time-dependent properties, which can not be computed with Monte Carlo simulations. This is how diffusion coefficients must be computed. It is also possible to use shearing boundaries in order to obtain a viscosity. Molec- 

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Jorgenson W L, Chandrasekhar J, Madura J D, Impey R W and Klein M L 1983 Comparison of simple potential funotions for simulating liquid water J. Chem. Phys. 79 926-35... 

It is also possible to simulate liquid droplets by surrouridiu g a solute by a fin ite ii urn ber of water moleeu les an d perform in g the sim -ulalion without a periodic box. The water, of course, eventually evaporates and moves away from the solute when periodic boundary con ditioii s arc n ot im posed. If the water is in itially added via periodic boundary con dition s, you rn ust edit the resu Itin g H IN file to remove th e periodic boti ruiary con ditioii s, if a droplet approach is desired. 

Using Monte Carlo to simulate liquids is reviewed in... 

Jorgensen, W.L. Chandrasekhas, J. Madura, J.D. Impey, R.W. Klein, M.L. Comparison of simple potential functions for simulation liquid water. J. Chem. Phys. 79 926-935, 1983. 

Alder and Wainwright gave MD treatments of particles whose pair potential was very simple, typically the square well potential and the hard sphere potential. Rahman (1964) simulated liquid argon in 1964, and the subject has shown exponential growth since then. The 1970s saw a transition from atomic systems... 

Comparison of Simple Potential Functions for Simulating Liquid Water William L. Jorgensen, Jayaraman Chandresekhar and Jeffrey D. Madura Journal of Physical Chemistry 79 (1983) 926... 

Jorgensen WL, Chandrasekhar J, Madura JD, Impey RW, Klein ML (1983) Comparison of simple potential functions for simulating liquid water. J Chem Phys 79(2) 926... 

Lekner J (1989) Summation of dipolar fields in simulated liquid vapor interfaces. Physica A 157(2) 826-838... 

Simulating Liquid Water near Mineral Surfaces Current Methods and Limitations... 

The most valuable of all the models of water, by far, is the computer simulated liquid with well defined water-water interaction. To date, molecular dynamics simulations for two pair potentials 3>, and Monte Carlo simulations for three pair potentials 7i>72>, have been published. The details of the methods of simulation can be found in the literature, to which the reader is referred. 

We conclude that care must be exercised lest characteristics of the computer simulated liquid which arise from specific details of the water-water potential used, and not from general features of that potential, be inferred to be character-... 

Schwope, A.D., Till, D.E., Ehntholt, D.J., Sidman, K. R., Whelan R.H., Schwartz P.S. and Reid R.C. (1987a). Migration of BHT and Irganox 1010 from low-density polyethylene (LDPE) to foods and food simulating liquids. Food Chem. Toxicol., 25, 4, 317-326. 

Akhand, N., Lapen, D. R., Topp, E., Edwards, M. J., Sabourin, L., Coelho, B. R., Duenk, P. W., Payne, M., and Gottschall, N. (2008). Using macro to simulate liquid sewage biosolid transport to tile drains for several land application methods. Trans. ASABE 51,1235-1245. 

KEMDS Process Simulation Liquid-liquid extraction hendrickson.html... 

The next stage - from theory to practice in distributed system development - is to include logistics planning and material flow simulation (liquid or gas). Based on virtual reality simulation systems the integrated simulation system will combine the virtual reality and the material distribution network simulation. 

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