[Pw_forum] Total energy diverges in cell relaxation

[Hui Tang]

Hi Axel,

I have used a much larger cutoff energy to rerun a problematic calculation.
This rerun does not have the divergence problem any more. I'm going to do a
few more runs to confirm this.

I think it's just as what you said that the effective cutoff energy becomes
smaller as the cell expands. So I probably should check the convergence of
stresses in my largest cell with respect to the number of plane waves.

btw, I did ghost states tests for my pseudo-potentials and there isn't any.


p.s., I remember to put on my affiliation this time. :)


Thank you very much,
---------------------------------------------
Hui Tang
Graduate Student
Department of Applied Physics
Yale University
Email: hui.tang at yale.edu
Phone: 1-203-432-7763



On Thu, Oct 16, 2008 at 4:00 PM, Axel Kohlmeyer <akohlmey at cmm.chem.upenn.edu
> wrote:

> On Thu, 16 Oct 2008, Hui Tang wrote:
>
> hi hui,
>
> your cell is expanding quite a bit and your cutoff is not
> that large. with an expanding cell, your _effective_ cutoff
> becomes much smaller as the number of plane waves remains
> constant during a variable cell calculation. have you tested
> the stess tensor convergence vs. the cutoff? you basically
> have to figure out how many plane waves you need for a well
> converged stress tensor at the largest expansion of your cell
> and then you have to set the cutoff at the beginning so that
> it results in the same number of plane waves...
>
> that would explain why it works (better) if you restart.
>
> also, did you test the pseudopotentials for ghost states?
>
> cheers,
>   axel.
>
> p.s.: please provide an affiliation. thanks.
>
> HT> Hi Nic,
> HT>
> HT> 1)  I have tested the pseudo-potentials (psps) with small atomic
> HT> separations. The pseudo-potentials give reasonable till two atoms are
> as
> HT> close as 0.8 A. When the total energy starts to diverge, which is about
> HT> Ionic iteration 17, the smallest inter-atomic distance is about 1.4 A.
> So I
> HT> do not think the breakdown of the psps is the trigger of this
> divergence of
> HT> the total energy, although it should be the reason for the enormous
> large
> HT> negative energies.
> HT>
> HT> 2) I have tried to restart the calculation from where the divergence
> starts.
> HT> Once I did that, the first scf calculation would give a reasonable
> energy
> HT> around -41.7 Ry, and the calculation could run smoothly till the total
> HT> energy converges most of the times. So what I deal with this problem
> right
> HT> now is to restart calculations from where they start to have diverged
> total
> HT> energies and it's been working well. However, I hope this is not the
> optimal
> HT> way to solve this problem. :)
> HT>
> HT> Thank you very much,
> HT> Hui
> HT>
>
> --
> =======================================================================
> 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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