While the realistically modeling of the thermodynamic behavior of fluids usually demands elaborated atomistic models, much have been learned from simplified ones. Here, we investigate a model where point-like particles (with activity z 0 ) are mixed with molecules that exclude their first and second neighbors (i.e., cubes of lateral size λ = √ 3a, with activity z 2 ), both placed on the sites of a simple cubic lattice with parameter a. Only hard-core interactions exist among the particles, so that the model is athermal. Despite its simplicity, the grand-canonical solution of this model on a Husimi lattice built with cubes revels a fluid-fluid demixing, yielding a phase diagram with two fluid phases (one of them dominated by small particles -F 0) and a solidlike phase coexisting at a triple-point. Moreover, the fluid-fluid coexistence line ends at a critical point. An anomaly in the total density (ρ T ) of particles is also found, which is hallmarked by minima in the isobaric curves of ρ T versus z 0 (or z 2 ). Interestingly, the line of minimum density cross the phase diagram starting inside the region where both fluid phases are stable, passing through the F 0 one and ending deep inside its metastable region, in a point where the spinodals of both fluid phases cross each other. arXiv:1907.01028v1 [cond-mat.soft] 1 Jul 2019

We study a generalized interacting self-avoiding walk (ISAW) model with nearest-and next nearest-neighbor (NN and NNN) interactions on the square and cubic lattices. In both dimensions, the phase diagrams show coil and globule phases separated by continuous transition lines. Along these lines, we calculate the metric ν t , crossover φ t and entropic γ t exponents, all of them in good agreement with the exact values of the Θ universality class. Therefore, the introduction of NNN interactions does not change the class of the ISAW model, which still exists even for repulsive forces. The growth parameters µ t are shown to change monotonically with temperature along the Θ-lines. In the square lattice, the Θ-line has an almost linear behavior, which was not found in the cubic one. Although the region of repulsive NNN interactions, with attractive NN ones, leads to stiff polymers, no evidence of a transition to a crystalline phase was found. * tiago@ufv.br

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