Soft core thermodynamics from self-consistent hard core fluids

In an effort to generalize the self-consistent Ornstein-Zernike approximation (SCOZA) -- an accurate liquid-state theory that has been restricted so far to hard-core systems -- to arbitrary soft-core systems we study a combination of SCOZA with a recently developed perturbation theory. The latter was constructed by Ben-Amotz and Stell [J. Phys. Chem. B 108,6877-6882 (2004)] as a reformulation of the Week-Chandler-Andersen perturbation theory directly in terms of an arbitrary hard-sphere reference system. We investigate the accuracy of the combined approach for the Lennard-Jones fluid by comparison with simulation data and pure perturbation theory predictions and determine the dependence of the thermodynamic properties and the phase behavior on the choice of the effective hard-core diameter of the reference system.Comment: 38 pages, 10 figure

Liquid-vapor transition of systems with mean field universality class

We have considered a system where the interaction, v(r) = v_IS(r) + xi^2 v_MF(r), is given as a linear combination of two potentials, each of which being characterized with a well-defined critical behavior: for v_IS(r) we have chosen the potential of the restricted primitive model which is known to belong to the Ising 3D (IS) universality class, while for v_MF(r) we have considered a long-range interaction in the Kac-limit, displaying mean field (MF) behavior. We study the performance of two theoretical approaches and of computer simulations in the critical region for this particular system and give a detailed comparison between theories and simulation of the critical region and the location of the critical point. Both, theory and simulation give evidence that the system belongs to the MF universality class for any positive value of xi and that it shows only non-classical behavior for xi=0. While in this limiting case theoretical approaches are known to fail, we find good agreement for the critical properties between the theoretical approaches and the simulations for xi^2 larger than 0.05.Comment: 9 pages, 11 figures, 3 table

Liquid-vapor transition of systems with mean field universality class

36 pagesWe have considered a system where the interaction, $v(r) = v_{\rm IS}(r) + \xi^2 v_{\rm MF}(r)$, is given as a linear combination of two potentials, each of which being characterized with a well-defined critical behavior: for $v_{\rm IS}(r)$ we have chosen the potential of the restricted primitive model which is known to belong to the Ising 3D (IS) universality class, while for $v_{\rm MF}(r)$ we have considered a long-range interaction in the Kac-limit, displaying mean field (MF) behaviour. We study the performance of two theoretical approaches and of computer simulations in the critical region for this particular system and give a detailed comparison between theories and simulation of the critical region and the location of the critical point. Both, theory and simulation give evidence that the system belongs to the MF universality class for any positive value of $\xi$ and that it shows only non-classical behavior for $\xi=0$. While in this limiting case theoretical approaches are known to fail, we find good agreement for the critical properties between the theoretical approaches and the simulations for $\xi^2$ larger than 0.05