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 > > >
