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[Scott Beckman <spbeckman@gmail.com>]

Hello,

I believe that the question is in regards to the rate of convergence -- 
i.e. how large of a basis is needed for the absolute convergence of the 
total energy, versus charge density, forces, etc.

According to the VASP user guide the total energy converges faster than 
forces, so even if the total energy is converged there remains 
so-called "Pulay stresses" due to the truncated basis set.  Pulay's 
paper on the subject was in regards to wavefunctions expanded in atomic 
orbitals not planewaves.

I've not systematically tested the convergence of forces and energy 
with respect to planewave cut-off, but it would be interesting to see 
such a study.

Scott


On Mar 29, 2006, at 12:11 PM, Michel Côté wrote:

> I do not understand your question very well. Toldff converge the 
> largest
> force. If you have convergence up to 10-5 (5 digits), then it is 
> normal that
> forces on atom that are smaller still change with increasing the 
> cutoff. In
> other words, it is not the number of significant digit that is 
> converged but
> the absolute number.
>
> Michel
>
>
> Le 29/03/06 10:41, « Nuno Galamba » <ngalamba@cii.fc.ul.pt> a écrit :
>
> > Dear Abinit users
> >
> > I have a simple question regarding the choice of ecut for an 
> > arbitrary system.
> >
> >  By doing a calculation for different values of ecut (say,
> > 10,15,20,25,30,35,40,45,50 Hartrees) for a given value of toldff (say 
> > 10-4
> > Ha/Bohr) one can  observe a plateau at which energy converges to.
> >
> > Now the question is, say the energy is converged for ecut=30 Ha 
> > (meaning it
> > does not change up to the first 5 digits), in principle the 
> > Hellmann-Feynman
> > forces on every particle of the system should be converged too.
> >
> > I am not quite sure this is the case. I observe that the force on the
> > particles of the system are not the same. These are of the order of 
> > 10-6
> > Ha/Bohr. Souldn't one expect some kind of convergence behavior for 
> > the force
> > on every particle when increasing systematically the number of plane 
> > waves
> > for a given calculation?
> >
> > Thanks in advance
>
>
> --
> ***************************************************************
>   Michel Cote                          tel: +1 (514) 343-5628
>   Professeur agrégé                   fax: +1 (514) 343-2071
>   Département de physique
>   Université de Montréal
>   C.P. 6128, succ. Centre-ville
>   Montréal (Québec) H3C 3J7          Michel.Cote@umontreal.ca
>   Canada                   www.phys.umontreal.ca/~michel_cote
> ***************************************************************