Dynamic Calculations of Molten Salt Structures

Molecular Dynamics Calculations of Molten Salt Structures 

It has already been seen in Seetion 2.17 that computer simulation of structures in aqueous solution can give rise to calculations of some static (e.g coordination numbers) and dynamic (e.g., diffusion coefficients) properties of ions in aqueous and nonaqueous solutions. One such computer approach is the Monte Carlo method. In this method, imaginary movements of the particles present are studied, but only those movements that /ower the potential energy. Another technique is molecular dynamics. In this method, one takes a manageable number of atoms (only a few hundred because of the expense of the computer time) and works out their movements at femtosecond intervals by applying Newtonian mechanics to the particles under force laws in which it is imagined that only pairwise interactions count. The parameters needed to compute these movements numerically are obtained by assuming that the calculations are correct and that one needs to find the parameters that fit. 

Experimental determinations and bench-type research may continue for a generation or two, for both methods need experimental results as a standard to judge success. As the variety of available software grows and beeomes more user friendly, the choice will move toward computer simulation. 

Some typieal radial distribution funetions for molten salts (see Section 5.23.2) are shown in Fig. 5.11. Those plots are made by assuming pairwise interaetions 

They naively assume that the additivity of such interactions (and nothing from ions outside the pairs) will allow a realistic calculation of physical properties. However, a somewhat more mature view is to take into account the dipole moments induced among ions by nearest neighbors. Since this changes the ions shapes (e.g., makes them spheroids and not spheres), the interaction equation usually assumed is not adequate. 

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