Evaluating Computational Shortcuts in Supercell-Based Phonon Calculations of Molecular Crystals: The Instructive Case of Naphthalene.

Tomas Kamencek,Sandro Wieser,Hirotaka Kojima,Natalia Bedoya-Martínez,Rochus Schmid,Johannes P Dürholt,Egbert Zojer

doi:10.1021/acs.jctc.0c00119

Abstract

Phonons crucially impact a variety of properties of organic semiconductor materials. For instance, charge- and heat transport depend on low-frequency phonons, while for other properties, such as the free energy, especially high-frequency phonons count. For all these quantities one needs to know the entire phonon band structure, whose simulation becomes exceedingly expensive for more complex systems when using methods like dispersion-corrected density functional theory (DFT). Therefore, in the present contribution we evaluate the performance of more approximate methodologies, including density functional tight binding (DFTB) and a pool of force fields (FF) of varying complexity and sophistication. Beyond merely comparing phonon band structures, we also critically evaluate to what extent derived quantities, like temperature-dependent heat capacities, mean squared thermal displacements, and temperature-dependent free energies are impacted by shortcomings in the description of the phonon bands. As a benchmark system, we choose (deuterated) naphthalene, as the only organic semiconductor material for which to date experimental phonon band structures are available in the literature. Overall, the best performance among the approximate methodologies is observed for a system-specifically parametrized second-generation force field. Interestingly, in the low-frequency regime also force fields with a rather simplistic model for the bonding interactions (like the General Amber Force Field) perform rather well. As far as the tested DFTB parametrization is concerned, we obtain a significant underestimation of the unit-cell volume resulting in a pronounced overestimation of the phonon energies in the low-frequency region. This cannot be mended by relying on the DFT-calculated unit cell, since with this unit cell the DFTB phonon frequencies significantly underestimate the experiments.

Highlights

Over the past years, molecular crystals have been the subject of numerous studies aimed at a better understanding of their properties in order to improve their performance in organicsemiconductor-based devices
In the following discussion we will separately benchmark the performance of the different methodologies for the low-frequency region and for the entire spectral range in which vibrations occur
This, for example, applies to the mean squared thermal displacement (MSTD) ⟨|uiα|2⟩ of atom α in direction i with mα being the mass of that atom

Summary

Introduction

Molecular crystals have been the subject of numerous studies aimed at a better understanding of their properties in order to improve their performance in organicsemiconductor-based devices Many of these properties are crucially influenced by phonons. For materials as complex as molecular crystals, it is difficult to reliably determine phonon bands both in experiments and in simulations: inelastic neutron scattering, which is typically used to measure phonon band structures, requires large single crystals, which are often difficult to grow These crystals ought to consist of deuterated molecules, as this results in higher coherent and lower incoherent scattering cross sections for neutrons.[15−17] As a consequence, to the best of our knowledge, the only crystal consisting of π-conjugated molecules for which experimental phonon band structure data are available is deuterated naphthalene.[18]

Methods

Results

Conclusion