[Pw_forum] problem with vc-relax

[Axel Kohlmeyer]

On Wed, 7 Jan 2009, loc duong ding wrote:

LDD> Dear all,
LDD> 
LDD> I have a problem when I run vc-relax input file. 
LDD> The force is increasing when the energy is decreasing. 
LDD> I wonder this is the normal case or not.   

in addition to what else was mentioned, you should check
whether your stress tensor is well converged with respect
to wavefunction and density cutoff. if this is not the 
case your system can behave very strange and you have factor
in that your "effective" cutoff is increasing with a shrinking
cell and decreasing with an expanding cell.

[...]

LDD> But It until runs well. I wonder the result for this calculation is 
LDD> believable or not.

another general remark. the fact that a calculation completes
only means that the computations could be completed without
errors in the input and without numerical instabilities. it
says very little about whether you calculation has physical
meaning. this is something _you_ have to find out and that 
can require some rather indirect tests and checks. scientific
codes - and quantum espresso is no exception - do not "know"
physics but only apply the math that has been programmed.
parameters and "symptoms" that may be acceptable for one 
calculation, can indicate big problems from some other.

the best way is to start by trying to experiment with calculations
where you know the result of and learning how to get it right
and how different parameters affect the correctness of the result.

cheers,
   axel.

LDD> 
LDD> Best regards,   
LDD> -----------------------------------------------
LDD> Loc Duong Dinh
LDD> Ms-Ph.D Student
LDD> Sungkyunkwan Advanced Institute of Nanotechnology,
LDD> Sungkyunkwan University,
LDD> Suwon, 440-746, Korea 
LDD> Email: mambom1902 at yahoo.com
LDD> 
LDD> 
LDD> 
LDD>       
LDD> _______________________________________________
LDD> Pw_forum mailing list
LDD> Pw_forum at pwscf.org
LDD> http://www.democritos.it/mailman/listinfo/pw_forum
LDD> 

-- 
=======================================================================
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
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.