[Pw_forum] problems with vc-relax

[Xijun Wang]

On Wed, Sep 7, 2011 at 3:28 PM, Elie Moujaes <elie.moujaes at hotmail.co.uk> wrote:
> Dear all,
>

Dear Elie,

>
> I am relaxing a 60 atom supercell (graphene grain boundary) to make the
> total force on atoms mainly 0.001 Ry/au. However the vc-relax stopped after
> 4 days of execution and without reaching the desired force threshold. The
> following output was obtained :
>
> Please can anyone advice me on this? Shall I start with the new
> configuration and start relaxing again?
>

I'm also doing some cell relaxation. As I understand, cell
optimization is not very easy, and it is common that it does not
converge after a very long run.

Surely you should restart the calculation to achieve your convergence
criteria. You can just change the "restart_mod = 'from_scratch' " to
"restart_mode = 'restart' ", and submit the job again to continue from
where you stopped. Please be advised that pw.x will read cell
parameters / atomic positions, initial density, pseudopotentials from
the folder of GBphonon.save, so make sure you copy the folder to
computing-nodes.

However, I highly suggest you do something to accelerate the
convergence. For example, if you do not expect the cell angle to
change (i.e. keep angles=90), you may add the following keywords to
the &CELL section.

   cell_dofree = xyz

This will greatly reduce the uncertainty of the geometry optimization.
and reduce energy/force oscillation.

Another great idea is to manually choose x, y and z, then do a
relaxation of atomic positions only. and plot the total energy changes
with x, y and z. However, be noted that different volume means
different numbers of PW basis function, and energy comparison in this
way could be a little confusing. Anyway, if volume change is not
large, still this would help a lot.

Since I'm also a learner, any more comments or correction to my
comments are welcome. Thanks

Xijun

>
> Regards
>
>
>
> Elie Moujaes
>
> University of Nott
>
> University Park
>
> NGT 3RD
>
>
>
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