Except for ab initio methods such as Car-Parrinello molecular dynamics1 (CPMD), which generate an intermolecular potential “on the fly,” the choice of intermolecular potential is a critical assumption that is made a priori for a molecular simulation.2 In the case of simulations involving only one component, the choice is relatively straight forward, and usually involves using the potential that best represents the properties of the pure fluid. If a common intermolecular potential is valid for different atoms or molecules, the study of multicomponent mixtures is likewise simple. In such cases the only additional decision is the choice of combining rules2 for the contributions of interactions between non-identical components to the intermolecular parameters. The use of combining rules can also be extended to mixtures involving components with different intermolecular potentials providing there is some degree of commonality. For example, there is no difficulty in calculating the non-identical interactions of a biomolecule modelled by the CHARMM3 force field and water represented by the simple point charge4 (SPC/E) model because 12-6 Lennard-Jones (LJ) interactions are common to both potentials and combining rules can be applied to obtain the non-identical LJ parameters. However, Increasingly, the accurate investigation of mixture properties will require the use of different intermolecular potentials for the various components. To the best of our knowledge the literature provides almost no guidance concerning the use of non-identical potentials.
We have investigated5 general methods for combining interactions between particles characterised by non-identical intermolecular potentials. The combination methods were tested by performing molecular dynamics simulations to determine the pressure, energy, isochoric and isobaric heat capacities, thermal expansion coefficient, isothermal compressibility, Joule-Thomson coefficient and speed of sound of 10-5 + 12-6 Mie6 potential binary mixtures. In addition to the two non-identical Mie potentials, mixtures are also studied with non-identical intermolecular parameters. The combination methods are compared with results obtained by simply averaging7 the Mie exponents. When either the energy or size parameters are non-identical, very significant differences emerge in the thermodynamic properties predicted by the alternative combination methods. The isobaric heat capacity is the thermodynamic property that is most affected by the relative magnitude of the intermolecular potential parameters and the method for combining non-identical potentials. Either the arithmetic or geometric combination of potentials provides a simple and effective way of performing simulations involving mixtures of components characterised by non-identical intermolecular potentials, which is independent of their functional form.
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