[Pw_forum] question on potential energy in constant pressure simulations

[Konstantin Kudin]

 Hi Nicola,

 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.

 Kostya


--- 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
> bright
> 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...).
> 
> 
> 
> 			nicola
> 
> 
> 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
> modified
> > kinetic energy functional such that I should disregard the
> potential
> > energy values and trust the stress instead ?
> > 
> >  Thanks!
> >  Kostya
> > 
> > 
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> -- 
> ---------------------------------------------------------------------
> 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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