Displaying phonon dispersion and density of states

Phonon dispersion curves show how phonon energy depends on the q-vector, along high symmetry directions in the Brillouin zone. This information can be obtained experimentally from neutron scattering experiments on single crystals. Such experimental data are available for only a small number of materials and thus, theoretical dispersion curves are useful for both establishing the validity of a modeling approach and to demonstrate the predictive power of ab initio calculations.

In certain circumstances it is possible to measure the density of states (DOS) rather than the phonon dispersion. Furthermore, the electron-phonon interaction function, which is directly related to the phonon DOS can be measured directly in the tunneling experiments (see, for example, Yanson, 1988). It is therefore important to be able to calculate phonon DOS from first principles, in order to compare the results with experimentally observed spectra.

Materials Studio can produce phonon dispersion and DOS charts from any .phonon CASTEP output file. These files are hidden in the Project Explorer but a .phonon file is generated with every .castep file that has a PhonDisp or PhonDOS suffix. All the files that can be used in the analysis of the current calculation will be presented in the results file selector. However, it is not recommended that you create phonon DOS plots based on the results of a phonon dispersion calculation that only uses q-vectors in high-symmetry directions.

When evaluating phonon DOS, use only the results of the phonon calculations on the Monkhorst-Pack grid.

To create a phonon dispersion chart

Choose Modules | CASTEP | Analysis from the Materials Studio menu bar.
Select Phonon dispersion from the list of properties.
Use Results file selector to pick the appropriate results file (it should have a PhonDisp suffix).
Select the required frequency units for the chart from the Units dropdown list.
Select a required display style (Points or Line) from the Graph style dropdown list.
Optionally, check Show DOS and set the relevant DOS options.
Click the View button.
A new chart document, seedname Phonon Dispersion.xcd, is created in the results folder.

To create a phonon DOS chart

Choose Modules | CASTEP | Analysis from the Materials Studio menu bar.
Select Phonon density of states from the list of properties.
Use Results file selector to pick the right results file (it should have a PhonDOS suffix).
Select a required DOS type (Full or Partial) from the DOS display dropdown list.
Optionally, set additional CASTEP Phonon DOS analysis options. The Interpolation Integration method gives more accurate results than the Smearing method, although it is slightly slower.
Click the View button.
A new chart document, seedname DOS.xcd, is created in the results folder when Full density of states is selected. A document seedname PDOS.xcd, is created in the results folder when Partial density of states is selected.

Displaying phonon dispersion and density of states

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