[Pw_forum] stress or total energy

[Javad hashemifar]

Dear PWscf users

I am trying to relax the c lattice parameter of a tetragonal supercell
that its a lattice parameter is fixed to some experimental value.
Therefore I am calculating this supercell at different values of c
(celldm(3)) and looking at the total energy and stress.
My feeling is that these two parameter should have consistent behavior
and both could be used to find the optimized c. I mean I expect that
when total energy is minimized, then the stress should vanish but it
is not the case for my calculation and I observe that when the total
energy is minimum, there is considerable value of stress (~ -10 kbar)
in the system.
I would appreciate if some body explain what is the problem and
if it is really true that stress and total energy have not the same
behavior, which one should be used to find out relaxed lattice
parameters. I expect that total energy is the main criteria for
relaxation of the lattice parameters ??

Best regards
Javad Hashemifar

On 12/27/06, Axel Kohlmeyer <akohlmey at cmm.chem.upenn.edu> wrote:
> On 12/25/06, Eduardo Ariel Menendez P
> <emenendez at macul.ciencias.uchile.cl> wrote:
> > Hi Paolo,
> > I saw these options in the examples of molecular dynamics woth PW. There
> > is few information in the Doc files and the forum.
> >
> >     pot_extrapolation='second-order',
> >     wfc_extrapolation='second-order',
> >
> > I guessed that extrapolation options would reduce the number of
> > self-consistent steps in MD steps. However, in the case I am running
> > (CdTe) he number of self consistent steps at each ionic steps averages
> > 16, regardless of the extrapolation keywords.
> >
> > Other keyword that I see in the examples is
> >     disk_io='high',etc,
> > Is this keyword useful to improve the efficiency of molecular dynamics?
>
> that depends on the individual example and hardware.
> with a higher disk_io level, more data is written to disk
> and re-read instead of being kept in memory. if you have
> a lot of memory, a lower disk_io level should be better.
> in fact, for some machines with no local disk storage,
> it would be nice of have a disk_io level of 'no', i.e. keep
> everything in memory...
>
> >
> > Other issue. I had this error in step 90
> >      iteration #  4     ecut=    30.00 Ry     beta=0.70
> >      Davidson diagonalization with overlap
> > %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
> >      from rdiaghg : error #      1986
> >      info =/= 0
>
>
> did you try changing the diagonalizer?
>
> > Consulting the manual and the forum I see that it is difficult to find
> > the cause, and that the first choice is restart
> > and see what happens. I did it and the calculation just continued without
> > error. ?Does it allows to discard the possible cause of bad
> > pseudopotential, and bug of the lapack algorithm, in favor of buggy mahine
> > optimization of lapack (MKL) ?
>
> i don't think that you have a buggy MKL. it might just be, that it get fed an
> input that it cannot converge. the fact that you do not see any improvements
> from wavefunction extrapolations could indicate that the order of your states
> keeps changing a lot and perhaps the davidson implementation is more
> sensitive there.
> (note: i'm not an expert here, but currently trying to learn about the details
> of these issues myself).
>
> i noticed however, that with different lapack implementations, this kind of
> error message keeps popping up more or less often (e.g. more often with
> ATLAS than with ACML than with MKL). also the compiler seems to have
> some impact, so it might be that very small differences in accuracy may
> have a large impact here (which would make it difficult to rule out all other
> possible sources of problems like suboptimal pseudopotentials, as they
> tend to enhance existing accuracy problems...).
>
> ciao,
>    axel.
>
> >
> > Thanks
> > Eduardo
> >
> > Eduardo A. Menendez Proupin
> > Department of Physics
> > Faculty of Science
> > University of Chile
> > Las Palmeras 3425
> > Ñuñoa, Santiago
> > Chile
> > Phone: 56+2+978 74 11
> > http://fisica.ciencias.uchile.cl/~emenendez/
> > _______________________________________________
> > Pw_forum mailing list
> > Pw_forum at pwscf.org
> > http://www.democritos.it/mailman/listinfo/pw_forum
> >
> >
>
>
> --
> =======================================================================
> 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.
> _______________________________________________
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>


-- 
======================================
Seyed Javad  Hashemifar,    Ph.D.
[current:]
Tel:+49-203-3794743        Fax:+49-203-3794742
Fachbereich Physik (AG Kratzer), Universitat Duisburg-Essen
47048 Dusiburg, Germany
[permanent:]
Tel:  +98-311-3912375       Fax: +98-311-3912376
Physics Department, Isfahan University of Technology
84156 Isfahan, Iran
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