Molecular simulations of benzene and hexafluorobenzene using new optimized effective potential models: Investigation of the liquid, vapor–liquid coexistence and supercritical fluid phases Dimitris Dellis ⁎, Ioannis Skarmoutsos, Jannis Samios Physical Chemistry Laboratory, Department of Chemistry, University of Athens, Panepistimiopolis 15771, Athens, Greece abstract article info Available online 8 May 2009 Keywords: Benzene Hexafluorobenzene Supercritical Local density inhomogeneities Force field Molecular dynamics Monte Carlo The development of new effective intermolecular potential models of benzene and hexafluororbenzene, capable in reproducing the thermodynamic and structural properties of molecular systems in a wide range of thermodynamic state points has been presented and discussed. Subsequently, the properties of the fluids have been investigated by employing molecular dynamics and Monte Carlo simulation techniques. The main purpose of this study was to reveal information concerning the liquid state, vapor–liquid equilibrium and supercritical phase properties of these fluids. In the case of the supercritical phase, we mainly focused on the behavior of local density inhomogeneities and related properties. Our calculations reveal that the local density augmentation is much more pronounced in the case of hexafluorobenzene. The origins of possible resemblances and discrepancies with available experimental data have been also systematically discussed and related to our conclusions reported in previous publications. The local density reorganization dynamics as a function of the bulk density and the size of the local region have been also studied, revealing a significant density and length scale dependence similar to the ones presented for other pure supercritical fluids in previous publication of our group. © 2009 Elsevier B.V. All rights reserved. 1. Introduction It is widely accepted nowadays that molecular simulation is one of the most powerful tools in predicting a wide range of properties of fluid systems, exhibiting thus very significant applications in chemistry, physics, materials science and biology [1–3]. Up to now, the majority of the scientific community is mainly using classical molecular simulation techniques (molecular dynamics or Monte Carlo) to investigate fluid phase properties. An extensive presentation of methods and techniques employed to study all these properties by classical molecular simulation can be found in many books and review articles [1–3] and refs therein. A key point on fluid phase classical molecular simulation is the selection of the appropriate molecular force fields, in order to provide a more realistic description of fluid properties in a wide range of thermodynamic conditions. It is common knowledge that the majority of the existing force fields have been parameterized to describe mainly liquid state properties and their transferability in different fluid phases has not been widely tested. Therefore the development of methods which ensure, more or less, the transferability of classical force fields in several fluid phases becomes indispensable. One additional and very important reason for this is also the necessity to provide realistic force fields to investigate the properties of condensed matter systems exhibiting some peculiar characteristic properties usually observed in much more extreme thermodynamic conditions than the ambient ones. A characteristic example of such type of fluids is the case of supercritical fluids (SC) and their mixtures (e.g. with ionic liquids). SC fluids represent one of the most interesting categories of solvents used in a wide range of applications in several fields like green chemistry, materials science, engineering etc [4–6]. For this reason, especially the last two decades, many research groups have focused their attention on the properties of SC fluids and many publications and reviews have been devoted to their special characteristics. According to the literature many interesting properties of SC fluids, like their high compressibility values in the near-critical region, have been attributed to some peculiar structural effects occurring in these fluids. It is widely known now that the structure of a SC fluid does not resemble that one of a homogeneous fluid and significant density inhomogeneities may be observed [6]. These distinct fluctuations in density are maximized in the thermodynamic phase region close to the critical point, causing a corresponding maximization of the isothermal compressibility factor k T . A systematic investigation of these important phenomena occurring in SC fluids might shed some light on many open questions upon their special characteristics, and molecular simulation is one of the most appro- priate techniques to do so. Therefore, the importance of developing Journal of Molecular Liquids 153 (2010) 25–30 ⁎ Corresponding author. E-mail address: ntell@chem.uoa.gr (D. Dellis). 0167-7322/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2009.04.007 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq