[Q-e-developers] Dielectric constant using PHONON, with and without GAMMA tricks

[Marco Govoni]

Dear QE team, 
I get something weird when I compute the macroscopic dielectric tensor of a SiH4 molecule. 
This is done using PWscf + PHonon (I tried both 4.3.2 and the SVN version of QE)
Here comes the problem:
If I use phcg.x (on top a SCF calculation with KPOINT(gamma) ), I get epsilon_XX = 4.565220 .
If I use ph.x (on top a SCF calculation with KPOINT(automatic) 111 000 ), I get epsilon_XX = 4.966717159 .
Why the two numbers are different? As far as I understand gamma_tricks should not affect so much the result. 
Please find attached the INPUT and OUTPUT files 
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Thanks.

Marco Govoni
UC Davis