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For a vibration freq. calc.(IBRION=5) POTIM is required to set the displacement size, right? What are the units on POTIM?

For an O2 or an H2 species if I relaxed all the degrees of freedom through POSCAR I should still be getting one real freq. corresponding to the true vibrational mode and the other 5 modes should be 0 I assume?

--the units of the displacement size is Ã…
--if you do not fix at least one atom, there will also be the degrees of motion of the molecule itself.
--concerning vibrational frequencies of H2: please note that H is treated as a classical atom as well, neglecting the (quantum effect) zero point vibrations. However, H is so light, that one should treat it as a quantum particle. When you do so, the average bond length increases somewhat, since the potential energy surface is not exactly quadratic (anharmonic effect).