# Modeling the structural and thermal properties of loaded metal-organic frameworks. An interplay of quantum and anharmonic fluctuations

## TL;DR

This study combines quantum and anharmonic simulations to accurately model the thermal and structural properties of loaded MOF-5, revealing property-dependent effects and providing insights for material design.

## Contribution

It introduces an accelerated simulation scheme and an optimized force field for efficient, accurate modeling of MOF-5 with guest molecules considering quantum and anharmonic effects.

## Key findings

- Thermal properties are well described by quantum harmonic models.
- Adsorbate behavior is classical and strongly anharmonic.
- Full quantum and anharmonic treatment is necessary for heat capacity predictions.

## Abstract

Metal-organic frameworks show both fundamental interest and great promise for applications in adsorption-based technologies, such as the separation and storage of gases. The flexibility and complexity of the molecular scaffold poses a considerable challenge to atomistic modeling, especially when also considering the presence of guest molecules. We investigate the role played by quantum and anharmonic fluctuations in the archetypical case of MOF-5, comparing the material at various levels of methane loading. Accurate path integral simulations of such effects are made affordable by the introduction of an accelerated simulation scheme and the use of an optimized force field based on first-principles reference calculations. We find that the level of statistical treatment that is required for predictive modeling depends significantly on the property of interest. The thermal properties of the lattice are generally well described by a quantum harmonic treatment, with the adsorbate behaving in a classical but strongly anharmonic manner. The heat capacity of the loaded framework - which plays an important role in the characterization of the framework and in determining its stability to thermal fluctuations during adsorption/desorption cycles - requires, however, a full quantum and anharmonic treatment, either by path integral methods or by a simple but approximate scheme. We also present molecular-level insight into the nanoscopic interactions contributing to the material's properties and suggest design principles to optimize them