Isomorphs and Pseudoisomorphs in Molecular Liquid Models

This thesis investigates new generic methods of identifying isomorphs and pseudoisomorphs in molecular viscous liquids using computer simulation. Isomorph theory has been studied for the last ten years in the Glass and Time group at Roskilde University. Strongly correlating liquids, so-called Roskilde simple liquids, are found to have invariant curves in their phase diagram, termed as isomorphs. The structure and dynamics of Roskilde simple liquids are invariant along isomorphs. These curves exist not only in atomic liquid models but also in molecular systems. Isomorphs usually are deter-mined along configurational adiabats and also via the direct isomorph check method. In this work, we introduce new generic "force methods" to generate isomorphs in rigid bonded models and pseudoisomorphs in harmonic spring bonded models.The atomic force method is introduced by Schrøder for the first time; it has been shown that this method works properly for the Kob-Andersen binary Lennard-Jones system. In this thesis, we introduce the molecular force and the torque methods which identify isomorphs in three small molecular models: asymmetric dumbbell, symmetric inverse power law (IPL) dumbbell and Lewis-Wahnström o-terphenyl (OTP). The advantage of force methods is that only a single configuration is required to predict isomorphs via these methods. The ability of these methods to predict isomorphs also are tested on a larger molecular model, i.e. flexible Lennard-Jones chains model with constraint bonds. In addition to the atomic and molecular forces,the segmental force is also investigated in the Lennard-Jones chains model.It has been shown that harmonic spring bonded molecular models have a poor correlation between the constant-density equilibrium virial and potential-energy fluctuations, and subsequently they are not supposed to have isomorphs. In 2016, Olsen et. al found invariant structure and dynamics in harmonic models, behaving like isomorphs which are termed pseudoisomorphs [2]. Olsen et. al used a challenging method to determine pseudoisomorphs, while we suggest much easier methods, ("force methods") to identify pseudoisomorphs in harmonic small and large molecular models. Two different scaling approaches, i.e. atomic scaling and center-of-mass scaling, are used to scale the configurations. We quench the system in order to generate pseudoisomorphs in harmonic models at high densities. Different minimization schemes are presented and developed to find the local minima. At low densities, pseudoisomorphs are identified in harmonic models via the force methods without quenching.The molecular force method can be considered as a general method to identify isomorphs and pseudoisomorphs in small molecular models at low and high densities. However, the force methods do not predict the isomorphic points in long flexible Lennard-Jones chains model along the large density change; but it is possible to identify pseudoisomorphs via atomic
and segmental forces in this model.