# FAQ Phonons

**Q.** How can I calculate phonons in a spin-polarized system?

**A.** Use the finite displacement method with or without a supercell. Spin-polarized DFPT is not yet available.

**Q.** How can I do a phonon calculation with DFT+D?

**A.** CASTEP currently supports semi-empirical dispersion corrections for phonons using both finite displacement (including supercell) methods and DFPT.

**Q.** My Raman calculation gives huge numbers or nonsense or "*" characters for the intensity. Why?

**A.** Astersisks ("*") instead of numbers is the traditional Fortran response when the number is too large to fit in the field. In this case it is almost certainly caused by a very poorly converged calculation giving numerical nonsense.

**Q.** How can I perform a "mode-following" calculation explore the energy of a function of amplitude for a particular phonon mode?

**A.** Use the *mode_follow* utility which comes with academic source distributions. This reads a *<seed>*`.phonon`

file from a previous calculation and generates a sequence of *<seed>-nn*`.cell`

files. `mode_follow --help`

gives some basic help information.

**Q.** I performed a phonon DOS calculation. Do I need to repeat all that calculation if I want dispersion curves as well?

**A.** No. If you used either the supercell or interpolation methods, the full force constant matrix is saved in the checkpoint file and can be used to recalculate phonons at any set of q. Just set up a "continuation" calculation to start from this file, and change the `PHONON_FINE_XXX`

keywords in the .cell file

**Q.** How can I generate a phonon DOS with both sharp and curved features as in textbook examples? I just get a wiggly mess from dos.pl.

**A.** Use the new *adaptive broadening* method in CASTEP 7.0. This is selected by keyword `PHONON_CALCULATE_DOS`

, and customized by `PHONON_DOS_SPACING`

, and generates a *<seed>*`.phonon_dos`

output file containing the precalculated DOS. This file can be plotted using dos.pl.

**Q.** How can I calculate and print a partial phonon density of states?

**A.** The built-in phonon DOS calculation activated by the `PHONON_CALCULATE_DOS`

keyword also computes a species-projected density of states.