[QE-users] difference in lattice parameters with USPP and ONCV NC PPs

[José Xavier]

Dear users, 

I'm trying to obtain the properties of the crystal of a drug (ibrav = -12). 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 give lattice parameters longer than experimental ones. So, I changed to the US PPs downloaded from the QE website (also PBE), and most of the values obtained after VC-relax were longer (<1%) than the experimental (as I expected from the literature).

Since I want to perform calculations to get the optical and vibrational properties of the crystal, I should use the NC PPs. However, I'm afraid it could give me the wrong vibrational/optical results. So, I would like to make some questions.

1- The input file was the same, So what could be the reason for that (small) difference?

2- Despite the small difference between ONCV NC PPs and US PPs for my lattice parameters, continuing using the first PP could give me a wrong result in the next calculations?

3- Is it "wrong" to use the USPPs to perform the vc-relax calculation and the ONCVNC for the other calculations?

4- Are there other ways to perform optical/vibrational/thermodynamic calculations by using the USPP or PAW PPs? I read superficially about BoltzTrap, but I don't know if it is possible.

Thanks for your time and help.

Sincerelly, 

José Xavier
UFRN - Brazil