Hii hifive users
I have done DFT calculations using VASP and PHONOPY to calculate phonon dispersion but there is small negative frequencies in that. Please guide me how to proceed with this data to use hipfive.
Please consult the ase documentation on how to handle input from vasp. There are also several tutorials on our documentation page which might be of interest.
For you specific problem; negative frequencies can manifest due to several reasons. Most common is that an un-relaxed primitive cell is used. It can also be due to acoustic sum-rules not being enforced or, in the case of 2D-materials, rotational sum rules. Yet another reason is using to small of a supercell.
Lastly, if you suspect that the material is unstable at 0K you can try the self-consistent phonon approach as documented in the literature and on our hompage.
Hope this help!
/Fredrik