Isomorphs in model molecular liquids

TL;DR

This paper extends the concept of isomorphs to rigid molecular liquids, demonstrating invariance of key properties along isomorphs and identifying master isomorphs in two molecular models.

Contribution

It generalizes isomorph theory to rigid molecules and compares two models, revealing invariance properties and the existence of master isomorphs.

Findings

Invariance of heat capacity, entropy, and scattering functions along isomorphs.

Instantaneous temperature and density jumps between isomorphic states cause no relaxation.

Lewis-Wahnstrom OTP model's isomorphs are less accurate than asymmetric dumbbell model's.

Abstract

Isomorphs are curves in the phase diagram along which a number of static and dynamic quantities are invariant in reduced units. A liquid has good isomorphs if and only if it is strongly correlating, i.e., the equilibrium virial/potential energy fluctuations are more than 90% correlated in the NVT ensemble. This paper generalizes isomorphs to liquids composed of rigid molecules and study the isomorphs of two systems of small rigid molecules, the asymmetric dumbbell model and the Lewis-Wahnstrom OTP model. In particular, for both systems we find that the isochoric heat capacity, the excess entropy, the reduced molecular center-of-mass self part of the intermediate scattering function, the reduced molecular center-of-mass radial distribution function to a good approximation are invariant along an isomorph. In agreement with theory, we also find that an instantaneous change of temperature and density from an equilibrated state point to another isomorphic state point leads to no relaxation. The isomorphs of the Lewis-Wahnstrom OTP model were found to be more approximative than those of the asymmetric dumbbell model, which is consistent with the OTP model being less strongly correlating. For both models we find "master isomorphs", i.e., isomorphs have identical shape in the virial/potential energy phase diagram.