Assessing the Performance of Density Functional Theory Methods on the Prediction of Low-Frequency Vibrational Spectra
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Assessing the Performance of Density Functional Theory Methods on the Prediction of Low-Frequency Vibrational Spectra Peter A. Banks1 · Zihui Song1 · Michael T. Ruggiero1 Received: 23 January 2020 / Accepted: 5 May 2020 / © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract The low-frequency (terahertz) dynamics of condensed phase materials provide valuable insight into numerous bulk phenomena. However, the assignment and interpretation of experimental results require computational methods due to the complex mode types that depend on weak intermolecular forces. Solid-state density functional theory has been used in this regard with great success, yet the selection of specific computational parameters, namely the chosen basis set and density functional, has a profound influence on the accuracy of predicted spectra. In this work, the role of these two parameters is investigated in a series of organic molecular crystals, in order to assess the ability of various methods to reproduce intermolecular forces, and subsequently experimental terahertz spectra. Specifically, naphthalene, oxalic acid, and thymine were chosen based on the varied intermolecular interactions present in each material. The results highlight that unconstrained geometry optimizations can be used as an initial proxy for the accuracy of interatomic forces, with errors in the calculated geometries indicative of subsequent errors in the calculated low-frequency vibrational spectra, providing a powerful metric for the validation of theoretical results. Finally, the origins of the observed shortcomings are analyzed, providing a basic framework for further studies on related materials. Keywords Density functional theory · Molecular dynamics · Vibrational spectroscopy · Low frequency · Crystalline disorder
Peter A. Banks and Zihui Song contributed equally to this work. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10762-020-00700-7) contains supplementary material, which is available to authorized users. Michael T. Ruggiero
[email protected] 1
Department of Chemistry, University of Vermont, 82 University Place, Burlington, VT 05405, USA
International Journal of Infrared and Millimeter Waves
1 Introduction Terahertz time-domain spectroscopy is a powerful method for the evaluation of crystalline phonons, which are increasingly being shown to be related to the bulk properties of the solids [1–4]. Low-frequency vibrations in molecular crystals often involve motions of entire molecules, for example hindered translations, making the vibrational dynamics dependent upon inherently weak long-range forces [1, 3, 5– 9]. This ultimately results in each molecular crystal, including crystals consisting of the same molecule but with different bulk packing (e.g., polymorphism or disorder), exhibiting a unique terahertz spectrum—both in mode type and frequency [3, 10–13]. But while terahertz time-domain spectroscopy is increasingly becoming more widely utilized, the assignment of the lo
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