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

[Ngoc Linh Nguyen]

Amandeep Kaur wrote:
> 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. 
Normally, a negative optical frequency specifies a case of unstable 
configuration.
> 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.
The problem possibly comes from non-synchronization of vc-relax and scf 
calculation functional, and it could result that the optimal geometry of 
vc-relax is not the optimal geometry of scf calculation.
> 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.
>
>   
I suggest that you should recalculate and synchronize functional in both 
vc-relax and scf, which is done by either keeping these parameters or 
removing them for both calculations.

Good luck

Linh
> Any help will be appreciated.
>
> Thanks
>
>
>