Mechanics and Atomistic Simulations

The techniques collectively termed molecular mechanics (MM) employ an empirically derived set of equations to describe the energy of a molecule as a function of atomic position (the Born—Oppenheimer surface). The mathematical form is based on classical mechanics. This set of potential energy functions (usually termed the force field) contains adjustable parameters that are optimized to fit calculated values of experimental properties for a known set of molecules. The major assumption is, of course, that these parameters are transferable from one molecule to another. Computational efficiency and facile inclusion of solvent molecules are two of the advantages of the MM methods. 

Another application of atomistic simulations is reported by De Pablo, Laso, and Suter. Novel simulations for the calculation of the chemical potential and for the simulation of phase equilibrium in systems of chain molecules are reported. The methods are applied to simulate Henry s constants and solubility of linear alkanes in polyethylene. The results seem to be in good agreement with experiment. At moderate pressures, however, the solubility of an alkane in polyethylene exhibits strong deviations from ideal behavior. Henry s law becomes inapplicable in these cases. Solubility simulations reproduce the experimentally observed saturation of polyethylene by the alkane. For low concentrations of the solute, the simulations reveal the presence of pockets in the polymer in which solubility occurs preferentially. At higher concentrations, the distribution of the solute in the polymer becomes relatively uniform. 

In the same vein, a configurational sampling technique was described that allows phase equilibria simulation of alkanes with longer chains. Several systems of linear alkanes at various temperatures were studied, and results compared to those from more conventional techniques show good agreement. 

Hutnik, Argon, and Suter have also used MM to investigate the molecu- 

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