26 How do I calculate the frequency dependent dielectric constant of a material?

Use what='scf_ph'. See example16 and example20. Please note that this calculation is made by the extended version of the phonon code available in thermo_pw, so you have to provide specific flags to the phonon input. See the user's guide for details. The dielectric constant is in the file output_epsilon.ps. Note that for metals you need to specify a finite wave-vector q and you cannot plot the dielectric constant when q is the $\Gamma$ point. See example17 and example21 for this case.