Abstract Monodisperse, purely repulsive hard spheres (MPRHS) are a canonical model for fluid–solid phase behavior in atomic and colloidal systems. Liquid‐state theory, free‐energy calculations, simulations, and experiments establish a first‐order fluid–solid transition in this model, a thermodynamic picture we take as given. Following Alder and Wainwright, we treat explicit fluid–crystal phase separation—coexistence of fluid‐and‐crystal domains—as an important benchmark for confirming first‐order behavior, complementary to demonstrating phase transition between single phases. Against this backdrop, we highlight a specific gap. Decades of simulations have mapped equations of state, coexistence properties, and nucleation rates, forming a foundational body of results, yet spontaneous, long‐lived fluid–crystal coexistence has not been reported in unbiased MPRHS simulations. Instead, coexistence appears either when physical/model‐level bias is introduced (e.g., seeding, gravity) or under algorithmic bias designed to accelerate barrier crossing. Studies that avoid bias typically observe transient mixed states ultimately overtaken by a single metastable phase, consistent with Frenkel's estimate that a spontaneous coexistence state in even large simulations would require years of sampling. In this Perspective, we focus on why such coexistence is so difficult to realize under pristine conditions, and what that means for testing Frenkel's entropy‐exchange mechanism in simulation. We argue that this kinetic difficulty is precisely what one expects from Frenkel's picture of competing vibrational and configurational entropy in hard‐sphere crystallization, and that minimal, controlled “hardness” perturbations that directly enhance in‐cage vibrational entropy provide the most direct route to making that entropic mechanism dynamically accessible and quantifiable.
Wang et al. (2026) studied this question.