Phonon Calc at Gamma

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Tiger-paw

Phonon Calc at Gamma

#1 Post by Tiger-paw » Sun Dec 04, 2005 9:27 pm

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
Last edited by Tiger-paw on Sun Dec 04, 2005 9:27 pm, edited 1 time in total.

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Phonon Calc at Gamma

#2 Post by admin » Mon Dec 05, 2005 9:59 am

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.
Last edited by admin on Mon Dec 05, 2005 9:59 am, edited 1 time in total.

dfcox
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Phonon Calc at Gamma

#3 Post by dfcox » Sat Feb 11, 2006 7:40 pm

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
Last edited by dfcox on Sat Feb 11, 2006 7:40 pm, edited 1 time in total.

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Phonon Calc at Gamma

#4 Post by admin » Mon Feb 13, 2006 12:46 pm

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
Last edited by admin on Mon Feb 13, 2006 12:46 pm, edited 1 time in total.

Tiger-paw

Phonon Calc at Gamma

#5 Post by Tiger-paw » Fri Sep 15, 2006 4:10 pm

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!
Last edited by Tiger-paw on Fri Sep 15, 2006 4:10 pm, edited 1 time in total.

Tiger-paw

Phonon Calc at Gamma

#6 Post by Tiger-paw » Fri Sep 15, 2006 4:15 pm

Sorry, "adsorbent" should be "adsorbates", i.e. adsorbed atoms or moleculers.
Last edited by Tiger-paw on Fri Sep 15, 2006 4:15 pm, edited 1 time in total.

bandy

Phonon Calc at Gamma

#7 Post by bandy » Sat Sep 16, 2006 6:51 am

why you want to study adsorbates on both sides(surfaces) of slab? Is there any specific reason? because the calculation will be very expensive?
Last edited by bandy on Sat Sep 16, 2006 6:51 am, edited 1 time in total.

Tiger-paw

Phonon Calc at Gamma

#8 Post by Tiger-paw » Thu Sep 21, 2006 3:48 pm

surface dipoles on two sides can be cancelled.
Last edited by Tiger-paw on Thu Sep 21, 2006 3:48 pm, edited 1 time in total.

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Phonon Calc at Gamma

#9 Post by admin » Tue Sep 26, 2006 11:35 am

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
Last edited by admin on Tue Sep 26, 2006 11:35 am, edited 1 time in total.

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