Thanks for your great software and your kind support in this community.
I am working on phonon calculation of UO2 with two U vacancies recently. I have created the 5 x 5 x 5 supercell fractional coordinates with lammps and converted it into GULP input format, as you can see in the attached gin file. I output the frequencies by using output command in the GULP input script. What I expect is the 3*N phonon frequencies at gamma point. However, it outputs several negative frequencies, which mean imaginary models, as explained in the gout file. It seems like supercell does matter to the phonon calculation, since I have tested single cell and got zero negative frequencies. Is there any suggestions from you?
Thanks for your kind support.