My Community

1.
To calc Phonon Freq in a crystal supercell at Gamma, I use IBRION=50, NFREE = 2, but how to choose a reasonable POTIM? Will the calculated phonon freq be sensitive to POTIM?

2.
In my calc, all freq I obtained for Gamma are nonzero. Am I supposed to see 3 zero freq and 3N-3 nonzero normal mode freq? What can I do for this?

Many thanks for your help,

tiger-paw

1) The Hessian Matrix is constructed via displacements from the equilibrium positons as set by POTIM, therefore the valuehas to be small enough to ensure that you are in the harmonic regime (a few hundereths of Ã…).
btw: IBRION =5 (not 50)
2) please choose a reasonable input in POSCAR (selective dynamics, T/F): if all positions are free to be displaced, the number of degrees of freedom is 3N, including the translation of the whole species as well.

Admin-

Could you be a little more specific about point 1) when IBRION=5? What value of POTIM is appropriate to give a displacement of a few hundreths of an Angstom? Does POTIM represent the magnitude of the displacement in Angstroms?

Thanks

1) please read the manual (http://cms.mpi.univie.ac.at/vasp/vasp/node101.html) concerning the
valid settings of IBRION.
2) POTIM is the displacement in Ã… you are right

Admin-

Could the direct/supercell phonon code be used to obtain all normal mode freqs for a surface slab, with vacuum and even adsorbent on both sides, and hence total free energy? (assuming all atoms are free to be displaced)

Is Gamma-point-only enough (or exact) for this C_1 sysmetry surface slab?

Many thanks!

Sorry, "adsorbent" should be "adsorbates", i.e. adsorbed atoms or moleculers.

why you want to study adsorbates on both sides(surfaces) of slab? Is there any specific reason? because the calculation will be very expensive?

surface dipoles on two sides can be cancelled.

to correct for the dipol moments, you can also set IDIPOL=3 if it helps to spare computing time
in principle, you can leave all atoms free to relax and calculate all vibration modes, however, this will be very time-consuming