Thermal Expansion in Dispersion-Bound Molecular Crystals

TL;DR

This study investigates how anharmonicity, nuclear quantum effects, many-body dispersion, and Pauli repulsion influence the thermal expansion of dispersion-bound molecular crystals, achieving accurate predictions of cell parameters and revealing significant quantum effects.

Contribution

It demonstrates that incorporating anharmonicity, NQE, many-body dispersion, and Pauli repulsion is essential for accurately modeling thermal properties of molecular crystals.

Findings

Anharmonicity yields cell parameters within 2% of experiments.

NQE cause a 40% volume increase in pyridine-I at 153 K.

Many-body dispersion and Pauli repulsion are crucial for predicting thermal expansivity.

Abstract

We explore how anharmonicity, nuclear quantum effects (NQE), many-body dispersion interactions, and Pauli repulsion influence thermal properties of dispersion-bound molecular crystals. Accounting for anharmonicity with $ab$ $initio$ molecular dynamics yields cell parameters accurate to within 2% of experiment for a set of pyridine-like molecular crystals at finite temperatures and pressures. From the experimental thermal expansion curve, we find that pyridine-I has a Debye temperature just above its melting point, indicating sizable NQE across the entire crystalline range of stability. We find that NQE lead to a substantial volume increase in pyridine-I ($\approx 40$% more than classical thermal expansion at $153$ K) and attribute this to intermolecular Pauli repulsion promoted by intramolecular quantum fluctuations. When predicting delicate properties such as the thermal expansivity, we show that many-body dispersion interactions and sophisticated treatments of Pauli repulsion are needed in dispersion-bound molecular crystals.