You should set the CASTEP task to "spectral", and then you set the special "spectral_task" to optics; i.e. in your param file you would have:

task : spectral

spectral_task : optics

You should also set a good spectral k-point sampling in your cell file, e.g. for a semiconductor you might have

spectral_kpoint_mp_spacing : 0.04

(For a metal you might need a finer sampling to describe the Fermi surface well. NB the default units are 2pi/Angstroms.)

This will calculate the Kohn-Sham ground state to get a good density using the standard k-point sampling in your cell (from "kpoint_mp_spacing" or whatever you used instead), then use that density to generate the Kohn-Sham eigenvalues on the finer grid specified by spectral_kpoint_mp_spacing. The eigenvalues are written to a ".bands" file and the optical matrix elements are written in an ome_bin or dome_bin file.

I recommend using the OptaDOS program to analyse the calculated optical data and plot it. A version of OptaDOS is included in the CASTEP source code distribution, but you can always get the latest version from github:

https://github.com/optados-developers/optados

Hope that helps,

Phil Hasnip

(CASTEP developer)

Your answer in very helpful. Thank you Philip James Hasnip