[Pw_forum] vc-relax

[Axel Kohlmeyer]

On Sat, 24 Jan 2009, Timothy Mason wrote:

TM> Dear Hania,
TM> 

TM> as someone else struggling with VC-relax, I'd like to see a grep of 
TM> your cell volumes.  (just grep on volume).  I find that in these 
TM> cases where the forces oscillate but the engergies decrease (all 
TM> cases for me) the volumes continue to expand about linearly with 
TM> time.  I'm not sure what to make of it.  It even happens starting 

good point. that could very likely mean, that your stress
tensor is not sufficiently converged wrt. to the wavefunction
or density cutoff. one of the most problematic issues with
vc-relax calculations is that the stress tensor need a much
higher wavefunction cutoff (and very much highe density cutoff
in case of ultra-soft pseudos) to be well converged. to make
matters worse, in a vc-relax calculations, you do not keep 
those cutoffs constant, but the number of plane waves. so for
an expanding cell you effectively decrease the cutoffs, and
if you stress tensor is not well converged, it will become
quickly not converged. there are some tricks to make this
effect less of a problem, but the immediate solution is to
start at a lattice constant that is definitely too large,
i.e. the cell will never expand beyond it. you still have
consider pulay stress, but that is relatively minor concern.

TM> with experimental structures often increasing them by 50 percent 
TM> before convergence thresholds are met.

a 50% increase sounds unphysical. have you tried restarting
from the final configuration (resets the "effective" basis 
set size) and see if the results are consistent?

cheers,
   axel.

TM> 
TM> would you please provide them?
TM> 
TM> Tim Mason
TM> University of Missouri St. Louis
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TM> 

-- 
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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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