Phase diagram of a double-occupancy cell model of a fluid with Curie-Weiss interaction

R. V. Romanik; O. A. Dobush; M. P. Kozlovskii; I. V. Pylyuk; M. A. Shpot

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arXiv · 2607.01009

Phase diagram of a double-occupancy cell model of a fluid with Curie-Weiss interaction

R. V. Romanik·O. A. Dobush·M. P. Kozlovskii·I. V. Pylyuk·M. A. Shpot

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Abstract

A double-occupancy cell model of a fluid with Curie-Weiss interaction is studied. First, we show that the model is isomorphic to the Blume-Capel model on a complete graph through a simple transformation from spin to occupancy variables. We then investigate its phase behavior within the grand-canonical ensemble using a combination of analytical and numerical methods. Despite its simplicity, the model exhibits a remarkably rich thermodynamic behavior depending on the ratio between the local repulsive and global attractive interactions. We identify regimes characterized by a single critical point, two distinct critical points, tricritical behavior, and triple-point formation. For sufficiently strong repulsion, the system possesses three fluid phases of different densities, leading to both gas-liquid and liquid-liquid coexistence. The locations of the critical, tricritical, and triple points are determined, and the corresponding phase diagrams are constructed. These results demonstrate that the competition between double-occupancy repulsion and long-range attraction is sufficient to generate complex phase behavior in a minimal multiple-occupancy lattice-gas model.

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R. V. Romanik, O. A. Dobush, M. P. Kozlovskii, I. V. Pylyuk, M. A. Shpot. 2026-07-01. Phase diagram of a double-occupancy cell model of a fluid with Curie-Weiss interaction. https://arxiv.org/abs/2607.01009

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