[Pw_forum] relaxation

[Axel Kohlmeyer]

On Tue, 29 May 2007, Jess Kondor wrote:

JK> Hi, everyone,

hi jess,

JK>  I have a question. Sometimes (at least, I found several times in the papers
JK> for ferroelectrics), we need to relax a system with very tight criteria for
JK> forces - up to 1 meV/Ang ( ~4.0d-5 in `pw (a.u.)` units). However, I found
JK> that it is hard to achieve this criteria. If I use 'bfgs' with   conv_thr =
JK> 1.d-6 (or better) and upscale = 10.d0 , calculations often crashes with

if you want to converge the geometry tightly, you have to converge 
the wavefunction even more tightly. also, you may run into 'ripples', 
particularly when using ultra-soft pseudopotentials.
in that case, you may also need to crank up the density cutoff.

cheers,
   axel.

JK> message ''bfgs history was reset on the previous step...' and number of bfgs
JK> steps = 15, number of scf steps = 2 . If I switch a relaxation method to
JK> 'damp' with dt=70.0, after one (or two) ionic steps forces are small, but
JK> then 'Total force' increases rapidly (from 0.001 to 0.4 or something) and
JK> remains almost constant. So, are there any other parameters which can help
JK> in geometry relaxation?
JK> 
JK> cheers,
JK>    Jess
JK> 

-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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