The PDielec Package

Authors: John Kendrick and Andrew Burnett

The Python package, PDielec calculates the infrared absorption and Raman scattering characteristics of a crystalline material supported in a non absorbing medium by post-processing the output of solid state quantum mechanical and molecular mechanical calculations of the phonons or dielectric response of the crystalline material. 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 has been published. Any use of the package should cite; PDielec: The calculation of infrared and terahertz absorption for powdered crystals John Kendrick and Andrew D. Burnett Journal of Computational Chemistry 2016, vol 27, 1491-1504 DOI: 10.1002/jcc.24344

PDielec also calculates powder and crystal Raman spectra, including polarisation and collection geometry. See the version 10 migration guide for changes to Python APIs, saved Raman geometry and spreadsheet comparisons.

Introduction

The molecular and solid state quantum mechanical calculations of response properties such as the frequencies and intensities of infrared (IR) and terahertz (THz) radiation absorption has become generally available in many molecular and solid state computer programs. A common approach is to assume the harmonic approximation and calculate the mass weighted force constant matrix (for molecules) or the dynamical matrix at the gamma point (for periodic solids). Diagonalisation of the matrix gives the frequencies for absorption and the normal modes (molecules) or phonon displacements (periodic solids). The calculation of the absorption intensity for each mode requires the calculation of the change in dipole moment caused by the displacement of the atoms for that mode. For solids where there is a large separation of charge, there can be a large coupling between a phonon mode and the internal field within a particle resulting from its morphology. The PDielec program is written in Python and post processes the output of solid state quantum mechanical (QM) and molecular mechanics (MM) based codes such as VASP, CASTEP, CRYSTAL23, Abinit, QuantumEspresso, Phonopy and GULP to predict the infrared absorption of crystalline insulator materials whose crystal size is small compared with the wavelength of the absorbing radiation. The package is suited for the calculation of the complex, frequency dependent permittivity and its associated absorption of infrared radiation for a finely ground crystalline material dispersed in a low loss dielectric medium such KBr or PTFE. A particular feature of the program is its ability to take into account the constant permittivity of the supporting medium and the particle shape through an effective medium theory. Calculation of ATR spectra has been included from v5.0. Version 10 introduced Raman scattering simulations. More details as to the theory are available here; https://johnkendrick.github.io/PDielec/

Installation

Full documentation of the program and its installation is available here; https://johnkendrick.github.io/PDielec/