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[Nuno Galamba <ngalamba@cii.fc.ul.pt>]

Dear Abinit users

Thanks for your replys on my question about force convergence.
I am aware that if you set toldff 10-4 Ha/Bohr then the energy will converge 
up to the 10-12 Ha. That clearly demonstrates that energy converges a lot 
faster than the forces for a given tolerance. My question was: Is there a 
toldff and an ecut that make my forces not to change if I increase ecut?
The implications of this are for example on the dynamics of a system. If for 
a 
given ecut and toldff the forces are not converged although the energy is, 
then on a MD simulation for example my dynamics can be "compromised" even 
though my energy is fully converged.

When using local basis functions, unless you use an extrapolation method for 
infinite basis sets, you are otherwise always working with non-conveged 
energies and forces. The great advantage of plane waves is exactly to avoid 
this problem, i.e., there are no Pulay forces and one can, in principle, work 
with converged energy (and forces ?).

I started doing some basic runs to get some insight on how fast energy and 
forces converge and was a bit surprised on that I cannot realy see a 
convergence pattern for the forces. 

The answer of Dr Michel Côté "If you have convergence up to 10-5 (5 digits), 
then it is normal that forces on atoms that are smaller still change with 
increasing the cutoff" unfortunately makes a lot of sense.

Following this, if the forces on my system are of the order of 10-6 Ha/Bohr 
then I need at least a toldff, say 10-9 Ha/Bohr, for a given value of ecut 
for which energy is converged, to avoid seeing my forces changing when 
increasing ecut. Is this what you meant ????

If so, then for most systems this is simply not possible. Then my question 
is: 
Does anybody know what the effect is on the dynamics of a system of working 
with non-converged forces??????

Thanks a lot once again

N. Galamba







On Wednesday 29 March 2006 19:11, 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