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

[Amandeep Kaur]

Thanks

If we remove those parameters in vc-relax calculation then it doesnt work
properly (non-zero pressure) . The scf calculation after using optimized
geometry gives almost zero pressure even if I dont use those parameters
(ecfixed, qcutz, q2sigma) but things change for phonon frequencies. I get
negative phonon frequencies (optical) by keeping those parameters inside the
scf calculation after which the phonon calculation follows.
*
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*Aman
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On Wed, Mar 3, 2010 at 12:28 AM, Ngoc Linh Nguyen <nnlinh at sissa.it> wrote:
> 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
>>
>>
>>
>
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-- 
Amandeep Kaur
Graduate Student
Department of Physics
University of California,Davis
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