Atomistic Simulations of Neat Ionic Liquids - Structure and Dynamics

2 Atomistic Simulations of Neat Ionic Liquids - Structure and Dynamics 

Atomistic simulations provide the positions (and in the case of MD, velocities) of aU the atoms in the system consistent with the equilibrium probability distribution 

The most common way in which liquid structure is quantified is through a radial distribution function (RDF), commonly given the symbol g(r). This function, which can be obtained experimentally, is a measure of the probability of observing two particular sites at a given distance from each other relative to a random distribution. The function is normalized such that when g(r) 1, there is a depletion of these pairsinthesystematadistancer,andwheng(r) 1, there is a relative enhancement of these types of interactions in the system. 

Margulis has shown [61] that as the alkyl chain length R increases from hexyl to dodecyl in [RMIM][PF6], the first and second solvation shells become slightly more ordered for the anion about the position of the cation. This same physical picture 

In general, a good force field can match experimental hquid densities vyithin 5% and derivative properties such as the volume expansivity and isothermal compressibility within 10-15% without recourse to parameter fitting. The more details included in the force field, the more accuracy can be ejq)ected. For example. Fig. 4.2-8 shows computed densities as a function of temperature for [BMIM][PF6]. UAl refers to a simple united atom force field developed by Shah et al. [90] that treats [PFe] as a sphere, UA2 is a different united atom potential that utilizes a seven-site model for [PFe] , [58] and AA is an aU-atom force field that exphdtly treats every atom in the system [11]. 

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