Wave vector dependent static dielectric properties of associated liquids: Methanol

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Abstract
The static dielectric properties of liquid methanol and their wave vector dependence have been investigated using a molecular dynamics (MD) computer simulation and the reference hypernetted chain (RHNC) integral equation formalism. In the MD simulation we have used the site–site interaction model of methanol proposed by Haughney et al. [M. Haughney, M. Ferrario, and I.R. McDonald, J. Phys. Chem. 91, 4934 (1987)] and we have adopted a simplified, spherical core, model of methanol in the RHNC calculation. We find that the static dielectric constant is equal to 24 and the Kirkwood factor is 1.9 for the MD model. Although these values are not in good agreement with the corresponding experimental ones (32 and 2.94, respectively), we are able to show that the size of our system is large enough such that its static dielectric properties are consistent with those of a real fluid. Both MD and RHNC results for the wave vector dependence of the static dielectric tensor show that the longitudinal component of the dielectric permittivity is negative for almost the entire range of wave vectors studied. We argue that there is nothing unusual about this seemingly curious behavior, and that it should in fact be common to most polar liquids. The consequences of this behavior for the evaluation of free energies of solvation are outlined.