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In my phonon calculation utilizing DFPT in VASP for a monolayer material with space group 187, I've noticed that while most of the phonon bands are positive, two of them appear to be straight. What could be the explanation for this observation?

Dear farah_shahzadi,

I am not quite sure what you mean by "straight" bands. Since you mention positive bands, do you mean that the problematic ones are negative (imaginary)? Please attach your input and output files (if possible also a plotted phonon band structure), so I can help you with your problem.

Cheers, Michael

Please find the picture attached.i am confused regarding the straight bands although overall bands are postive?what these straight bands tell?

Dear farah_shahzadi,

please post your input and output files, as well as any other pertinent information, in a compressed folder, as per our forum guidelines. Otherwise, I cannot interpret the data you present.

Cheer, Michael