[Pw_forum] Use of ecfixed, qcutz, q2sigma in a scf calculation following vc-relax

[Amandeep Kaur]

Hello

I did vc-relax calculation for ZrNCl and I got the optimized geometry.
Then I did a scf calculation using the relaxed coordinates and atomic
positions and I incorporated the ecfixed, qcutz and q2sigma in the scf
calculation. Then I did a phonon calculation using the same prefix as
in the scf calculation ( phonon calculation follows from a scf
calculation) and I got negative optical frequencies. Is this due to
the use of ecfixed, qcutz and q2sigma in the scf calculation because I
ran one scf calculation using the optimized geometry without using the
above parameters and did a phonon calculation following scf
calculation and there were no negative phonon frequencies.

Are these parameters (ecfixed, qcutz and q2sigma) only to be used for
vc-relax not for the scf calculation even if we want to use the
optimized geometry because the pressure from the scf calculation using
optimized geometry comes out to be small but not exactly the same as
that of vc-relax calculation.

Then the question is that should these parameters (ecfixed, qcutz and
q2sigma) be used or not for scf calculations because I got negative
optical phonon frequencies though I am using optimized geometry.

Any help will be appreciated.

Thanks


-- 
Amandeep Kaur
Graduate Student
Department of Physics
University of California,Davis