Stepwise Homogeneous Melting of Benzene Phase I at High Pressure

Abstract

We investigate, using molecular dynamics simulations, the spontaneous homogeneous melting of benzene phase I under a high pressure of 1.0 GPa. We find an apparent stepwise transition via a metastable crystal phase, unlike the direct melting observed at ambient pressure. The transition to the metastable phase is achieved by rotational motions, without the diffusion of the center of mass of benzene. The metastable crystal completely occupies the whole space and maintains its structure for at least several picoseconds, so that the phase seems to have a local free energy minimum. The unit cell is found to be unique—no such crystalline structure has been reported so far. Furthermore, we discuss the influence of pressure control on the melting behavior.

A Short Review of Current Computational Concepts for High-Pressure Phase Transition Studies in Molecular Crystals

“…Nevertheless, these methods are relatively rarely used for molecular crystals at high-pressure conditions and have a specific discussion in literature [51,52], thus would not be described further in this work. We also do not describe molecular dynamics (MD) methods which are very useful [53][54][55], having not much experience in these methods. Modern machine learning and big data approaches are also not specified here, showing significant progress in the crystal structure and properties prediction, but not being applied to high-pressure phase transitions in molecular crystals [56][57][58][59][60].…”

Pressure-Induced Phase Transformations

“…In such compounds, a small pressure of the order of a few GPa could have drastic consequences. In the contribution to this Issue, Mahesta and Mochizuki [7] investigate, using molecular dynamics simulations, the spontaneous homogeneous melting of benzene (C 6 H 6 ) under a pressure of 1.0 GPa. They propose the existence of an apparent stepwise transition via a metastable crystal phase, unlike the direct melting observed at ambient pressure.…”