The ABINIT Users Mailing List ( CLOSED )

["D. R. Hamann" <drh@bell-labs.com>]

Scott,

For a bit of clarification, there are no Pulay corrections to forces in 
a plane-wave basis, since the basis set does not depend on the postions 
of the atoms.  There is a Pulay correction to the stress, however.  
Abinit reduces this effect using esmooth, but does not, I believe, 
totally eliminate it.  In general, forces (1st derivatives of the 
energy) and force constants, etc. as calculated in the Abinit 
response-function mode (2nd derivatives) converge more slowly than the 
energy itself, but with the exception of stress and elastic constants, 
this is not a Pulay effect.

Don Hamann

Scott Beckman wrote:

> 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
> > ***************************************************************

--
D. R. Hamann                            Phone: 908-582-4454
Director, Theoretical Materials         Fax:   908-582-4868
 Physics Research (retired)             email: drh@bell-labs.com
Bell Laboratories 
Lucent Technologies
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