Exploring the Reliability of DFT Calculations of the Infrared and Terahertz Spectra of Sodium Peroxodisulfate
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Exploring the Reliability of DFT Calculations of the Infrared and Terahertz Spectra of Sodium Peroxodisulfate John Kendrick1 · Andrew D. Burnett1 Received: 3 July 2019 / Accepted: 30 October 2019 / © The Author(s) 2019
Abstract A number of DFT programs with various combinations of pseudo-potentials and van der Waals’ dispersive corrections have been used to optimize the structure of sodium peroxodisulfate, Na2 (SO4 )2 , and to calculate the infrared, attenuated total reflectance and terahertz absorption spectra of the powdered crystal. Comparison of the results from the different methods highlights the problems of calculating the absorption spectrum reliably. In particular the low frequency phonon modes are especially sensitive to the choice of grids to represent the wavefunction or the charge distribution, k-point integration grid and the energy cutoff. A comparison is made between the Maxwell-Garnett (MG) and Bruggeman effective medium methods used to account for the effect of crystal shape on the predicted spectrum. Possible scattering of light by air inclusions in the sample and by larger particles of Na2 (SO4 )2 is also considered using the Mie method. The results of the calculations are compared with experimental measurements of the transmission and attenuated total reflection spectra. Keywords Terahertz · Density functional theory · Spectroscopy · Infrared · Phonon
1 Introduction Infrared and terahertz (THz) spectroscopies are incredibly powerful analytical techniques with many applications across the physical and life sciences. Whilst the origin Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10762-019-00643-8) contains supplementary material, which is available to authorized users. Andrew D. Burnett
[email protected] John Kendrick [email protected] 1
Department of Chemistry, University of Leeds, Leeds, LS2 9JT, UK
International Journal of Infrared and Millimeter Waves
of many of the spectral features in an infrared spectrum can be easily interpreted with a knowledge about characteristic vibrational frequencies of functional groups, to identify the origin of all peaks (particularly below 1000 cm−1 ) and understand the subtleties in peak shape and position theoretical support is essential. There are now a number of density functional theory (DFT) based packages designed for both small molecules [1, 2] and periodic solids [3–7] capable of calculating vibrational frequencies and infrared intensities that can be used to interpret complex experimental spectra. There are also post-processing tools such as PDielec [8] which take into account effective medium approximations [9], the attenuated total reflection (ATR) effect [10] and Mie scattering [11] to aid in the interpretation of complex experimental spectra. Whilst these packages are now readily accessible, the calculation of spectra of complex systems, that correlate well with experiment can still be tricky, particularly at frequencies below 200 cm−1 [12–14]. Choice of basis set or pseudo-poten
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