Abstract-Exploring the Reliability of DFT Calculations of the Infrared and Terahertz Spectra of Sodium Peroxodisulfateghggg

John Kendrick, Andrew D. Burnett

https://link.springer.com/article/10.1007/s10762-019-00643-8

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.

Abstract-PDielec: The calculation of infrared and terahertz absorption for powdered crystals

John Kendrick, Andrew D. Burnett,
http://onlinelibrary.wiley.com/doi/10.1002/jcc.24344/full

The Python package PDielec is described, which calculates the infrared absorption characteristics of a crystalline material supported in a non-absorbing medium. PDielec post processes solid-state quantum mechanical and molecular mechanical calculations of the phonons and dielectric response of the crystalline material. Using an effective medium method, the package calculates the internal electric field arising from different particle morphologies and calculates the resulting shift in absorption frequency and intensity arising from the coupling between a phonon and the internal field. The theory of the approach is described, followed by a description of the implementation within PDielec. Finally, a section providing several examples of its application is given. 
© 2016 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.

Abstract-The effect of molecular size and particle shape on the terahertz absorption of a homologous series of tetraalkylammonium salts.

Andrew David Burnett , John Kendrick , Christopher Russell , Jeppe Christensen , John E Cunningham ,Arwen R. Pearson , Edmund H Linfield , and A. Giles Davies

http://pubs.acs.org/doi/abs/10.1021/ac401657r

The absorption coefficient and refractive index have been measured for a homologous series of tetraalkylammonium bromides over the frequency range 0.3 to 5.5 THz. Spectral features are found to shift to lower frequencies as the molecular mass is increased, as expected. However, in order to understand the detailed structure of the observed spectral features, density functional perturbation theory calculations have been performed on the first four crystalline compounds in the series. From these calculations we find that each spectrum is dominated by three translatory modes involving asymmetric motion of the ammonium cation and bromine counter-ion, although the overall number of active modes increases with increasing molecular size. The experimentally observed absorption is not completely described by the infrared active phonon modes alone. We show that it is also necessary to include the coupling of the phonon modes with the macroscopic field generated by the collective displacement of the vibrating ions and we have applied an effective medium theory which accounts for particle shape to allow for this effect in the calculation of the terahertz spectra.