[Pw_forum] vc-md question

[Paolo Giannozzi]

On Thursday 26 January 2006 18:03, Miguel Martínez Canales wrote:

> 2) On another ground, why are the stresses shown in a scf run and 
> in a vc-md run different?

they shouldn't be (and I am quite sure they aren't). Are you by any 
chance using the modified kinetic energy functional in vc-md runs?

> 3) Would a cp or fpmd run be better suited for my needs (good 
> relax for a future phonon calculation)?

I don't think so: you can find the optimized structure with a series 
of calculations at fixed volumes and c/a, if this is all you need.

> 4) VASP featured fixing some atoms. As far as I know, espresso 
> doesn't allow this (at least for pw.x runs). Am I totally wrong?

don't be so pessimistic: you are wrong on this specific point only :-)
See the definition of ATOMIC_POSITIONS , variables if_pos.

> Side note: I'm running ESPRESSO v3.0. Under espresso 2.1.5, 
> the Wentzkovitz lagrangian seemed to not evolve

there were some problems in previous versions with variable-cell 
dynamics, but they should have been fixed in 2.1.5, AFAIK.
Unfortunately nobody seems to be interested in maintaining
the variable-cell stuff. I am seriously considering removing it
altogether from future releases.

> while the Parrinelo-Raman lagrangian would not run unless
> nosym=.true. was stated. 

this is correct: since the Parrinello-Rahman may break the symmetry,
you cannot rely on symmetry to reduce the number of k-points

Paolo

-- 
Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
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