Stability of separation-shifted Lennard-Jones fluids

Ruelle's thermodynamic stability criteria are applied to the separation-shifted Lennard-Jones (SSLJ) fluid, and the domain of its parameters giving "normal" thermodynamic stability in the thermodynamic limit is established. Fluids interacting with the SSLJ both conforming to and breaking these stability criteria were modeled using molecular dynamics computer simulation. For system sizes typical of most simulations, the transition between the two patterns of behavior was found to be smeared out over a range of parameter values. Thermodynamic instability is marked by a collapse of the system into a small "ball" or volume. The collapsed state nevertheless has some statistical mechanical properties typical of systems exhibiting normal thermodynamics (e.g., the kinetic and configurational temperatures were found to be the same in the collapsed state within statistics).