[Pw_forum] Projection, convergence and imaginary phonon frequencies

[Axel Kohlmeyer]

On Mon, 12 Jan 2009, Nicola Marzari wrote:

NM> Dear John,

dear nicola,

happy new year from cold philly.

NM> if it is a crystal, the most critical parameter should be k-point
NM> sampling. Increase that, maybe using shifted meshes of the form
NM> 2n 2n 3n 1 1 1 , with n=1,2,3...

but it is a _molecular_ crystal, so i guess there are some 
areas with comparatively low electron density and quite weak 
interactions, as seen by the long list of rather soft modes.
i would suspect that will need to be converged very well wrt 
to the basis set to not see any ripples on the potential surface.

for TM oxygen pseudos i would go to 120ry or higher to get
tight convergence. for MD 80-85ry is usually sufficient 
(the more traditional 70ry has been shown to be very sloppy),
but if you want a tight geometry convergence you will have
to be more careful. BFGS can be quite sensitive and will 
essentially refuse to converge if you have "ripples" due
to a very flat potential surface unless you crank up the
cutoff significantly.

NM> Norm-conserving and ultrasoft should matter little (unless
NM> there is a problem with one pseudo), while GGAs should be a bit
NM> tougher to converge than LDA.
NM> 
NM> Do "crystal" and "simple" really lead to a different number of
NM> modes (10 vs 13) ??

those are obviously only the lowest few modes. 
it is a 9x9x6 angstrom cell!

cheers,
   axel.

NM> 
NM> 			nicola
NM> 
NM> 
NM> John Kendrick wrote:
NM> > Hi,
NM> > 
NM> > I have been doing some calculations recently on molecular crystals. 
NM> > Because of our interest in the lowest phonon frequencies I have been 
NM> > concerned that translational invariance is maintained as far as 
NM> > possible.  To this end I have increased the accuracy of the optimisation 
NM> > and the scf convergence, to a point where I cannot see how to converge 
NM> > the calculation any tighter.  I noticed that the norm conserving 
NM> > pseudopotentials were having greater difficulties than the others.  In 
NM> > particular for one molecule I have seen significant differences between 
NM> > the different projection schemes that ensure translational invariance. 
NM> > Particularly the intensities in the infrared a very different.  Below is 
NM> > some output from a representative calculation.
NM> 
NM> ---------------------------------------------------------------------
NM> Prof Nicola Marzari   Department of Materials Science and Engineering
NM> 13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
NM> tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu
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-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
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