[Pw_forum] questions on molecular dynamics example

[Axel Kohlmeyer]

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/
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>
>


-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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