Lattice thermal expansion and anisotropic displacements in urea, bromomalonic aldehyde, pentachloropyridine and naphthalene

TL;DR

This study evaluates the accuracy of first-principles lattice dynamics in predicting anisotropic displacement parameters, thermal expansion, and motion in four molecular crystals, showing improved agreement with experiments over harmonic models.

Contribution

The paper demonstrates that quasi-harmonic first-principles calculations reliably predict ADPs and thermal properties in diverse molecular crystals, highlighting the importance of anharmonic effects.

Findings

ADPs are accurately predicted up to 200 K using quasi-harmonic approximation.

Predictions agree better with experimental data than harmonic models.

Normal-mode anharmonicity influences ADP calculations.

Abstract

Anisotropic displacement parameters (ADPs) are commonly used in crystallography, chemistry and related fields to describe and quantify thermal motion of atoms. Within the very recent years, these ADPs have become predictable by lattice dynamics in combination with first-principles theory. Here, we study four very different molecular crystals, namely urea, bromomalonic aldehyde, pentachloropyridine, and naphthalene, by first-principles theory to assess the quality of ADPs calculated in the quasi-harmonic approximation. In addition, we predict both thermal expansion and thermal motion within the quasi-harmonic approximation and compare the predictions with experimental data. Very reliable ADPs are calculated within the quasi-harmonic approximation for all four cases up to at least 200 K, and they turn out to be in better agreement with experiment than the harmonic ones. In one particular case, ADPs can even reliably be predicted up to room temperature. Our results also hint at the importance of normal-mode anharmonicity in the calculation of ADPs.