[Pw_forum] about convergence criterion in supercell
Axel Kohlmeyer
akohlmey at cmm.chem.upenn.edu
Wed Jul 16 16:18:10 CEST 2008
On Wed, 16 Jul 2008, lfhuang at theory.issp.ac.cn wrote:
LFH> Dear all friends:
dear <insert name here>,
LFH> I have a question on convergence criterion in supercell, which is:
LFH>
LFH> For unit cell, I define "conv_thr" and "tr2_ph" to be 1*d-12 and
LFH> 1*d-14,respectively. Then I take a 4*4*4 supercell, and is it
LFH> precise enough to define 4*4*4*d-12 and 4*4*4d-14 for "conv_thr"
LFH> and "tr2_ph" respectively? If it is not, what is the reason?
perhaps you should first explain _your_ rationale for lowering
the convergence threshold when increasing the system size.
a convergence parameter that would depend this much on
the system size would not be a good choice.
in any case, the rationale to choose a specific convergence criterion
should be based on the convergence of the property that you are
interested in. i.e. if you want to do a geometry optimization, the
forces have to be converged accurately enough, not only with respect
to wavefunction/density-cutoffs, k-point sampling but also relative
to the wavefunction convergence. similarly, for other calculation types.
cheers,
axel.
LFH>
LFH> Thanks a lot for your time to read my letter!
LFH> And I would like greatly appreciate the one who can give me some resonable
LFH> rationale!
LFH>
LFH> Best Wishes!
LFH>
LFH>
LFH> _______________________________________________
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LFH>
--
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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
tel: 1-215-898-1582, fax: 1-215-573-6233, office-tel: 1-215-898-5425
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