Thanks <br><br>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. <div>
<b><span class="Apple-style-span" style="font-weight: normal;"><br></span></b></div><div><b><span class="Apple-style-span" style="font-weight: normal;">Aman<br></span></b><div><br>On Wed, Mar 3, 2010 at 12:28 AM, Ngoc Linh Nguyen <<a href="mailto:nnlinh@sissa.it">nnlinh@sissa.it</a>> wrote:<br>
> Amandeep Kaur wrote:<br>>> Hello<br>>><br>>> I did vc-relax calculation for ZrNCl and I got the optimized geometry.<br>>> Then I did a scf calculation using the relaxed coordinates and atomic<br>
>> positions and I incorporated the ecfixed, qcutz and q2sigma in the scf<br>>> calculation. Then I did a phonon calculation using the same prefix as<br>>> in the scf calculation ( phonon calculation follows from a scf<br>
>> calculation) and I got negative optical frequencies.<br>> Normally, a negative optical frequency specifies a case of unstable<br>> configuration.<br>>> Is this due to<br>>> the use of ecfixed, qcutz and q2sigma in the scf calculation because I<br>
>> ran one scf calculation using the optimized geometry without using the<br>>> above parameters and did a phonon calculation following scf<br>>> calculation and there were no negative phonon frequencies.<br>
> The problem possibly comes from non-synchronization of vc-relax and scf<br>> calculation functional, and it could result that the optimal geometry of<br>> vc-relax is not the optimal geometry of scf calculation.<br>
>> Are these parameters (ecfixed, qcutz and q2sigma) only to be used for<br>>> vc-relax not for the scf calculation even if we want to use the<br>>> optimized geometry because the pressure from the scf calculation using<br>
>> optimized geometry comes out to be small but not exactly the same as<br>>> 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.<br>
>><br>>><br>> I suggest that you should recalculate and synchronize functional in both<br>> vc-relax and scf, which is done by either keeping these parameters or<br>> removing them for both calculations.<br>
><br>> Good luck<br>><br>> Linh<br>>> Any help will be appreciated.<br>>><br>>> Thanks<br>>><br>>><br>>><br>><br>> _______________________________________________<br>> Pw_forum mailing list<br>
> <a href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a><br>> <a href="http://www.democritos.it/mailman/listinfo/pw_forum">http://www.democritos.it/mailman/listinfo/pw_forum</a><br>><br><br><br><br>-- <br>Amandeep Kaur<br>
Graduate Student<br>Department of Physics<br>University of California,Davis<br><br></div></div>