Phonon energies, infrared and Raman spectra

The study of the vibrational modes of matter (or phonons) is an important topic in condensed matter. These phonons are key components in order to understand thermal or electrical conductivity, heat capacity or sound propagation.

Density Functional Theory, by giving a reliable and unexpensive way of measuring the energy and forces of a system of interest, is often used to compute these normal modes. Experimentally, they are studied by infrared and Raman spectroscopy. The intensity of both types of spectroscopy differ for each mode and both can be studied from first principles.

Three workflows are implemented in MyBigDFT to allow the computation of these quantities:

• The Phonons class allows to compute these phonons, and especially their energies.

• The InfraredSpectrum class allows to compute the infrared spectrum (that is, measuring the energies and their related intensities). It requires the same number of DFT calculations used to compute the phonons: once the phonons are computed, the infrared intensities can be readily obtained from the variations of the dipole moments along these modes. No extra BigDFT calculations is therefore required.

• The RamanSpectrum class allows to compute the Raman spectrum. It does so at the expense of requiring even more DFT calculations, due to the need for the derivatives of the electronic polarizability tensor for each normal mode.

To get further details on each workflow, see the links below:

• Phonon energies
• Infrared spectrum
• Raman spectrum