Effects of Interaction Range on Fluid Multicriticality: A Computational Study of an Interconverting Lattice Model

Abstract: The range of intermolecular interactions plays a central role in determining the nature of phase behavior and critical phenomena. It is well established through studies of the Ising model that as interaction range increases, Monte Carlo simulations progressively approach meanfield predictions as the effects of critical fluctuations are suppressed. In this work, we investigate how varying interaction range influences fluid multicriticality using an interconverting lattice model that exhibits both Ising-like liquid-gas criticality and symmetric fluid tricriticality (similar to that in the superfluid $<sup>4$He-$<sup>3$He mixture). This minimal model serves as a representative system for exploring the evolution of competing critical points within a generic framework. We analyze the model using both meanfield theory and three-dimensional Monte Carlo simulations while systematically varying the number of interacting neighbors, $Z_n$, from 6 to 388. We find that the system with nearest-neighbor interactions ($Z_n=6$) reveals only two types of multicritical behavior, while for larger interaction ranges, four distinct archetypes emerge. We demonstrate the convergence of the simulation results to those of the meanfield theory as the number of interacting neighbors tends to infinity, and we discuss the results within the framework of crossover critical phenomena.