[Pw_forum] Total energy diverges in cell relaxation

[Hui Tang]

Hi Nic,

1)  I have tested the pseudo-potentials (psps) with small atomic
separations. The pseudo-potentials give reasonable till two atoms are as
close as 0.8 A. When the total energy starts to diverge, which is about
Ionic iteration 17, the smallest inter-atomic distance is about 1.4 A. So I
do not think the breakdown of the psps is the trigger of this divergence of
the total energy, although it should be the reason for the enormous large
negative energies.

2) I have tried to restart the calculation from where the divergence starts.
Once I did that, the first scf calculation would give a reasonable energy
around -41.7 Ry, and the calculation could run smoothly till the total
energy converges most of the times. So what I deal with this problem right
now is to restart calculations from where they start to have diverged total
energies and it's been working well. However, I hope this is not the optimal
way to solve this problem. :)

Thank you very much,
Hui

On Thu, Oct 16, 2008 at 3:09 PM, Nicola Marzari <marzari at mit.edu> wrote:

> Hui Tang wrote:
> > Dear all,
> >
> > I'm trying to do cell relaxation on my Mg-Boron systems. For some of the
> > structures, I got extremely strange results. The total energy diverges
> > starting from some ionic step. For example, here are the total energy in
> > each ionic step for one of the problematic run:
> >
>
> Two suggestions
>
> 1) are atoms overlapping much more than they should ? I.e. are these
> new configurations reasonable, or are core overlaps very significant ?
> The pseudopotential approximation breaks down if cores overlap (a lot).
>
> 2) is it a bug/memory overrun ? Take the first configuration with
> much lower energy, and rerun that from scratch. Do you get an
> energy that looks like -41.7 Ry, or much lower ?
>
>                        nic
>
>
>
> ---------------------------------------------------------------------
> Prof Nicola Marzari   Department of Materials Science and Engineering
> 13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
> tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu
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