Hi,
It might be because you have not geometry optimized your system completely. I recommend you to run a relax calculation with following parameters changed, they might seem overkill but they work: in CONTROL etot_conv_thr = 1.0D-6, forc_conv_thr = 1.0D-5, in SYSTEM nosym = .false., in ELECTRON diagonalization = 'cg', conv_thr = 1D-8, With a dense k-point grid, however as the symmetry is broken the k-point grid might be dense - you need to check. Then a vc-relax run with same parameters (and update atomic positions), but set cell_dofree as per your requirement. You also need to check your structure after the geometry optimization, there might be a change in angle from say 90 degrees to 89.9 degrees, this will also give you negative or imaginary frequences. I also recommend you to get the crystal structure data form: materialsproject.org They are generally optimized or with very fewer forces. Regards, Ghadiyali Mohammed Kader Dept. of Physics, University of Mumbai.
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