Liquid-like and gas-like features of a simple fluid: An insight from theory and simulation

A soft-sphere system is one of the simplest and most studied models of many-body problems. In contrast to real substances, it does not have a cohesive state and boiling line. For this reason, the fluid phase has been considered to be the same everywhere on the phase diagram with no difference between the gas and liquid phase. Here, we show that the soft-sphere fluid can in fact be demarcated into two states with liquid-like and gas-like properties even though these states are not separated by a thermodynamic phase transition. We find that particle dynamics and excitation spectra are qualitatively different in the two states. This difference applies to other system properties such as viscosity, diffusion and speed of sound. We consider different repulsion exponents of the soft-sphere system as well as the limiting case of the hard-sphere system and comment on their implications for demarcating the fluid phase into liquid-like and gas-like states.