[Pw_forum] question on potential energy in constant pressure simulations

[Nicola Marzari]

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
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