[Pw_forum] question on potential energy in constant pressure simulations
konstantin_kudin at yahoo.com
Thu Apr 20 18:20:40 CEST 2006
Thanks for the info! I was under the impression that ALL bugs were
fixed by now, but apparently - not. I do in fact use ultra-soft PSPs,
so that would explain my results.
An important consideration for me is that I did not intend just to
test the stress, but rather to use it for some production jobs. So
unfortunately it looks like this has to wait.
--- Nicola Marzari <marzari at MIT.EDU> wrote:
> Dear Kostya,
> great that your are testing the constant-pressure cp.
> There is still an unresolved issue (bug...) for ultrasoft psp, in
> the diagonal terms only - that could explain your results. On the
> side, a lot of other arcane issues (bugs) that had crept in the CVS
> have been recently found and fixed by Carlo C. and Paolo G.
> The safest first step would be to look at the numerical derivative of
> the ground-state energy with respect to your variable lattice
> vector - should be equal with the calculated stress
> (again, if you are using psp, it will not...).
> Konstantin Kudin wrote:
> > Hi all,
> > I am doing the CP dynamics with one of the lattice vectors being
> > optimized as well.
> > I am using a very recent CVS (yesterday's), which should have the
> > correct stress (knocking on wood).
> > The parameters for the modified kinetic energy functional are:
> > ecutwfc = 30.0, ecfixed = 25.0, qcutz = 25.0, q2sigma = 3.0
> > This is for what used to be just ecutwfc=25.0 in constant volume
> > simulations.
> > So what I am seeing is that while the stress becomes smaller, the
> > potential energy of the system goes up at the same time with the
> > decreasing stress.
> > Should not the decrease in stress correspond to the *decreasing*
> > potential energy? Or is there some funny business with this
> > kinetic energy functional such that I should disregard the
> > energy values and trust the stress instead ?
> > Thanks!
> > Kostya
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