[QE-users] How can I calculate optical properties and get the correct PPs?

[José Xavier]

Dear, 

I'd like to calculate the optical properties of a crystal structure. Most of the tutorials I've seen used the epsilon.x function. However, I read a comment that epsilon.x only works with NC pseudopotentials. 

At the beginning of this work, I was using the ONCV NC PPs from the SG15 database and PseudoDojo (PBE), but both PPs gave me lattice parameters shorter than the crystal values, after optimization. These values were <2% shorter, but most of the literature shows that GGA functionals always give lattice parameters longer than experimental ones. So, I changed to the US PPs downloaded from the QE website (also PBE), and the values obtained after VC-relax were longer (<1%) than the experimental (as I expected from the literature).

Could anyone help me with these problems:
1- Should I use the ONCV NC PPs to calculate all the properties (even with the shorter lattice parameters)? 
2- Are there other places/ways to get the NC PPs?
3- Are there other ways to calculate the optical properties, without using the NC PPs?

Thank you for the help

Sincerely,
José Xavier
Department of Biophysics and Pharmacology
Federal University of Rio Grande do Norte
Natal, Brazil