We need 3 files

si-optics.cell

%BLOCK LATTICE_CART
2.73  2.73 0.00
2.73  0.00 2.73
0.00  2.73 2.73
%ENDBLOCK  LATTICE_CART

%BLOCK POSITIONS_FRAC
Si 0.0     0.0     0.0
Si 0.25    0.25    0.25
%ENDBLOCK POSITIONS_FRAC

SYMMETRY_GENERATE

KPOINTS_MP_GRID 10 10 10  
SPECTRAL_KPOINTS_MP_GRID 14 14 14

si-optics.param

TASK                   : SPECTRAL
SPECTRAL_TASK          : OPTICS
CUT_OFF_ENERGY         : 200
IPRINT                 : 2

 

si-optics.odi

TASK               : optics
JDOS_SPACING       : 0.01
JDOS_MAX_ENERGY    : 30
EFERMI             : optados
DOS_SPACING        : 0.1
BROADENING         : adaptive # Default
ADAPTIVE_SMEARING  : 0.4     # Default
OPTICS_GEOM        : polycrystalline     # Default
OPTICS_LOSSFN_BROADENING : 0.0

Note

There are 2 ways to use the pseudopotentials:

- Using an external pseudopotential with extention .usp

- Using an internal pseudopotential created by the code during the execution according to type mentioned  in the param file which we will do it in this tutorial.


Execution

For serial calculation

/Si-optics$ castep.serial si-optics
/Si-optics$ optados si-optics 

For parallel calculation 

/Si-optics$ mpirun -np 4 castep.mpi si-optics

/Si-optics$ optados.mpi si-optics

 

We will get the following files

/Si-optics$ ls
Si_C19_LDA_OTF.usp  Si-optics_absorption.dat  Si-optics.cell  Si-optics_epsilon.agr Si-optics.odi   Si-optics_reflection.dat  Si-optics.0001.spec       Si-optics.check 
Si-optics_epsilon.dat  Si-optics.odo  i-optics.0002.spec   Si-optics.bands 
Si-optics_conductivity.agr    Si-optics.ome_bin         Si-optics_refractive_index.agr
Si-optics.0003.spec       Si-optics.bib     Si-optics_conductivity.dat  Si-optics_loss_fn.agr 
Si-optics-out.cell        Si-optics_refractive_index.dat Si-optics.0004.spec 
Si-optics.castep            Si-optics_loss_fn.dat  Si-optics.param  Si-optics_absorption.agr
Si-optics.castep_bin      Si-optics.cst_esp             Si-optics_reflection.agr

Plotting 

/Si-optics$ xmgrace Si-optics_epsilon.agr
/Si-optics$ xmgrace Si-optics_absorption.agr
/Si-optics$ xmgrace Si-optics_conductivity.agr
/Si-optics$ xmgrace Si-optics_loss_fn.agr
/Si-optics$ xmgrace Si-optics_reflection.agr
/Si-optics$ xmgrace Si-optics_refractive_index.agr
   

 We will get the following picture

 






Reference:   https://castep-docs.github.io/castep-docs/tutorials/Optics/Optics/