In this work,we use the discrete perturbation theory (Benavides,A.L.;Gil-Villegas,A.Mol Phys.1999,97,1225) to obtain a theoretical equation of state for the Jagla ramp potential.This potential is a spherically symmetric,two scale potential,with both repulsive and attractive ramps,that has been studied mainly because it models the thermodynamic behavior of single-component systems that exhibit liquid-liquid transitions and some of the water-type anomalies.The main advantage of the presented equation of state is that it is an analytical expression in the parameters that characterize the intermolecular interactions.This feature allows one to systematically study the effect of the variation of the parameters on the thermodynamic properties of this system.Besides,with a suitable choice of parameters,it is possible to model the behavior of a family of substances having common features.Single-phase internal energies and pressures at different conditions of density and temperature and some phase diagrams for several values of the intermolecular parameters are presented and compared with Monte Carlo NPT and Gibbs ensemble simulation data.We give a theoretical condition for the appearance of a density anomaly for second-order high-temperature perturbation theories with temperature-independent reference systems.We apply this equation of state to the case of water,giving a qualitatively correct vapor-liquid phase diagram.
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