Scaling Properties of Liquid Dynamics Predicted from a Single Configuration: Pseudoisomorphs for Harmonic-Bonded Molecules

TL;DR

This paper introduces three force-based methods to identify pseudoisomorphs in molecular models with harmonic bonds, demonstrating that internal degrees of freedom minimally impact the scaling of dynamical properties.

Contribution

The study presents novel force-based techniques to trace pseudoisomorphs from a single configuration in molecular models with harmonic bonds, expanding understanding of liquid dynamics scaling.

Findings

Different methods work for different molecular models.

Internal degrees of freedom have limited impact on scaling behavior.

Quenching reduces the influence of internal motions on dynamics.

Abstract

Isomorphs are curves in the thermodynamic phase diagram of invariant excess entropy, structure, and dynamics, while pseudoisomorphs are curves of invariant structure and dynamics, but not of the excess entropy. The latter curves have been shown to exist in molecular models with flexible bonds [A. E. Olsen et al., J. Chem. Phys. 145, 241103 (2016)]. We here present three force-based methods to trace out pseudoisomorphs based on a single configuration and test them on the asymmetric dumbbell and 10-bead Lennard-Jones chain models with bonds modeled as harmonic springs. The three methods are based on requiring that particle forces, center-of-mass forces, and torques, respectively, are invariant in reduced units. For each of the two investigated models we identify a method that works well for tracing out pseudoisomorphs, but these methods are not the same. Overall, it appears that the more internal degrees of freedom there are in the molecule studied, the less they affect the gross dynamical behavior. Moreover, the "internal" degrees of freedom (including rotation) do not appear to significantly affect the scaling behavior of the dynamical/transport coefficients provided some "quenching" is performed.