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
