[CPMD-list] Geometry optimization problem

[Axel Kohlmeyer]

On Mon, 14 May 2007, fafa chiker wrote:

FC> many thanks Pr. Axel,
FC> I have used the pseudopotentials of Goedecker(As-q5,
FC> B-q3, and Ga-q3), so I thought that the cutoff up to
FC> 50Ryd or 70Ryd is enough to get a good convergence.
FC> (the test of mesh cutoff have been done on supercells
FC> of host atoms). There are some conditions to respect
FC> in using Goedecker type pseudopotentials?????  

goedecker pseudopotentials are - due to how they are 
constructed - much 'harder' than troullier-martins potentials
and need thus need on average a significantly higher 
plane wave cutoff for converged forces. what is a sufficient
cutoff has to be evaluated on a per-atom basis. 

the advantage of goedecker potentials is, that they tend to
be more transferable, particularly for 'tricky' (heavy) elements.

i'm currently running some tests experimenting with goedecker 
potentials where one of the elements is oxygen (so a _very_ 'hard'
atom) and it turns out that a cutoff of 120ry, which is usually
ok to get a stable and sufficiently accurate MD, is not good
enough for getting clean and smooth potential scans for weak
interactions and even some 150ry scans are a bit 'jumpy', which
has a noticable effect on the result of geometry optimizations.

cheers,
   axel.


FC> 
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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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