Isomorphs are curves in the phase diagram along which both structure and dynamics to a good approximation are invariant. There are two main methods to trace out isomorphs in both atomic and molecular systems, the configurational adiabat method and the direct isomorph check method. We introduce and test a new family of force based methods on three molecular models; the asymmetric dumbbell model, the symmetric inverse power law dumbbell model, and the Lewis-Wahnström model of o-terphenyl. A unique feature of the force based methods is that they only require a single configuration to trace out an isomorph. The atomic force method was previously shown to work very well for the Kob-Andersen binary Lennard-Jones mixture, but we show that it does not work for molecular models. In contrast, we find that a new method based on molecular forces works well for all three molecular models.
Isomorphs are curves in the thermodynamic phase diagram along which structure and dynamics are invariant to a good approximation. There are two main ways to trace out isomorphs, the configurational-adiabat method and the direct-isomorph-check method. Recently a new method based on the scaling properties of forces was introduced and shown to work very well for atomic systems [Phys. Rev. Lett.2022129245501]. A unique feature of this method is that it only requires a single equilibrium configuration for tracing out an isomorph. We here test generalizations of this method to molecular systems and compare to simulations of three simple molecular models: the asymmetric dumbbell model of two Lennard-Jones spheres, the symmetric inverse-power-law dumbbell model, and the Lewis–Wahnström o-terphenyl model. We introduce and test two force-based and one torque-based methods, all of which require just a single configuration for tracing out an isomorph. Overall, the method based on requiring invariant center-of-mass reduced forces works best.
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