Toward a Reliable Description of the Lattice Vibrations in Organic Molecular Crystals: The Impact of van der Waals Interactions

Abstract

This work assesses the reliability of different van der Waals (vdW) methods to describe lattice vibrations of molecular crystals in the framework of density functional theory (DFT). To accomplish this task, calculated and experimental lattice phonon Raman spectra of a pool of organic molecular crystals are compared. We show that the many-body dispersion (MBD@rsSCS) van der Waals method of Ambrosetti et al. and the pairwise method of Grimme et al. (D3-BJ) outperform the other tested approaches (i.e., the D2 method of Grimme, the TS method of Tkatchenko and Scheffler, and the nonlocal functional vdW-DF-optPBE of Klimeš et al.). For the worse-performing approaches the results could not even be fixed by the introduction of scaling parameters, as commonly used for high-energy intramolecular vibrations. Interestingly, when using the experimentally determined unit cell parameters, DFT calculations using the PBE functional without corrections for long-range vdW interactions provide spectra of similar accuracy as the MBD@rsSCS and D3-BJ simulations.

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

“…For calculating -point phonons, density functional theory (DFT) 21,22 is typically the method of choice. Indeed, as shown recently for a variety of conjugated materials and their polymorphs, 23 combining DFT with a suitably chosen van der Waals correction, yields an excellent agreement with experimental Raman data even in the range between ~5 cm -1 and 100 cm -1 . As indicated above, the situation becomes computationally more challenging, as soon as phonons in the entire first Brillouin zone (1BZ) need to be considered.…”

“…Bearing in mind that the low-frequency vibrations are crucially impacted by intermolecular interactions (see above), for identifying the ideal reference methodology it is useful to first assess the performance of different a posteriori vdW correction schemes. In this context, it has been shown recently for -point vibrations (specifically for Raman spectra) that the D3-BJ correction yields highly accurate results for a variety of rather complex organic semiconductors and their polymorphs 23 . Essentially the same accuracy has been obtained in ref 23…”

Anharmonic Lattice Vibrations in Small‐Molecule Organic Semiconductors

“…We note that the apparent redshifts (e.g., in the higher‐energy part of the pentacene spectrum) can be explained by a tendency of underbinding in organic crystals when the here‐applied, many‐body dispersion (MBD) correction is used . In Section S2 in the Supporting Information, we demonstrate the superior performance of the MBD approach compared to using the regular Tkatchenko–Scheffler (TS) method …”

Crystal alignment of surface stabilized polymorph in thioindigo films

“…After re-determining, by X-ray crystallography, the structure of the thioindigo polymorphs, here named α and β, we have interpreted and rationalized their polarized Raman spectra in the low frequency range with the aid of DFT (Density functional theory) calculations. Over this range, polymorphs can be quickly and efficiently discriminated by identifying their vibrational fingerprint [10][11][12][13][14], a tool of the utmost importance when dealing with films. By growing micro-and nanocrystals on surfaces, using solution methods such as drop casting and https://doi.org/10.1016/j.dyepig.2019.107847 Received 17 July 2019; Received in revised form 28 August 2019; Accepted 28 August 2019 the solution bar assisted meniscus shearing (BAMS) technique [15][16][17], we have investigated the polymorph selectivity of the substrate.…”