A Numerical Test of a High-Penetrability Approximation for the One-Dimensional Penetrable-Square-Well Model

TL;DR

This study evaluates a high-penetrability approximation for a one-dimensional penetrable-square-well fluid, comparing analytical predictions with Monte Carlo simulations to understand phase behavior and structural properties.

Contribution

The paper introduces and tests a high-penetrability approximation for the 1D penetrable-square-well model, extending previous low-penetrability results and analyzing phase transition possibilities.

Findings

High-penetrability approximation aligns with Debye-Hückel theory.

No evidence of fluid-fluid transition in simulations.

Clustering transition occurs when Ruelle's stability criterion fails.

Abstract

The one-dimensional penetrable-square-well fluid is studied using both analytical tools and specialized Monte Carlo simulations. The model consists of a penetrable core characterized by a finite repulsive energy combined with a short-range attractive well. This is a many-body one-dimensional problem, lacking an exact analytical solution, for which the usual van Hove theorem on the absence of phase transition does not apply. We determine a high-penetrability approximation complementing a similar low-penetrability approximation presented in previous work. This is shown to be equivalent to the usual Debye-H\"{u}ckel theory for simple charged fluids for which the virial and energy routes are identical. The internal thermodynamic consistency with the compressibility route and the validity of the approximation in describing the radial distribution function is assessed by a comparison against numerical simulations. The Fisher-Widom line separating the oscillatory and monotonic large-distance behavior of the radial distribution function is computed within the high-penetrability approximation and compared with the opposite regime, thus providing a strong indication of the location of the line in all possible regimes. The high-penetrability approximation predicts the existence of a critical point and a spinodal line, but this occurs outside the applicability domain of the theory. We investigate the possibility of a fluid-fluid transition by Gibbs ensemble Monte Carlo techniques, not finding any evidence of such a transition. Additional analytical arguments are given to support this claim. Finally, we find a clustering transition when Ruelle's stability criterion is not fulfilled. The consequences of these findings on the three-dimensional phase diagrams are also discussed.