# [Pw_forum] Total energy vs PP and some beginner questions

Giuseppe Mattioli giuseppe.mattioli at ism.cnr.it
Fri Apr 10 12:14:40 CEST 2015

```Dear Konrad

> 1. To what extent has the pseudopotentials of two different elements
> must be the same? Can I use spin polarized together with none spin
> polarized PP? Or can I for example use Fe.pbe-sp-van.upf with
> Y.pbe-nsp-van.UPF? Or even combine for example Fe.pbe-sp-van.upf with
> Y.pbe-mt_fhi.UPF ?

There is no strict rule, you can combine different kinds of pseudopotential, as long as you know why you are using just "that" PP for "that" element.
Remember that the wfc and density cutoffs can be *very* different for different kinds (i.e., US, NC, PAW) of PP related to the same element.

> 2. I have noticed that combining some PP together gives very different
> total energies: for example for the same system first run with Fe and Y
> with ",pbe-mt_fhi.UPF" gives total = -200 Ry and when using
> Fe.pbe-sp-van.upf with Y.pbe-nsp-van.upf gives total energy = -1333 Ry.
> Why those two PP give so big difference?

Total energies are not meaningful when you use PPs, only energy differences, unless you are using PAW psudopotentials which "reconstruct" the core
contribution to total energy and may be tentatively used to compare the results with those obtained by all-electron codes.

> 3. The last one noob-like question:  While doing a 'relax' calc. only
> the X and Y coordinates change during calculation, the Z always is such
> as set on the begining. Is that normal? (or maybe it is only fortuity
> for my atomic system?)

This is fortuitous. It probably means that forces along z are 0 by symmetry in your system.

HTH

Giuseppe

On Friday, April 10, 2015 11:33:00 AM Konrad Gruszka wrote:
> Dear Users,
>
> Recently while making some tests on simple systems I've found very
> strange behaviour of pw.x. It is obvious that choosing different pseudo
> potentials should give to some extent different total energies. Since
> I'm a beginner,  I would therefore ask a few questions:
>
> 1. To what extent has the pseudopotentials of two different elements
> must be the same? Can I use spin polarized together with none spin
> polarized PP? Or can I for example use Fe.pbe-sp-van.upf with
> Y.pbe-nsp-van.UPF? Or even combine for example Fe.pbe-sp-van.upf with
> Y.pbe-mt_fhi.UPF ?
>
> 2. I have noticed that combining some PP together gives very different
> total energies: for example for the same system first run with Fe and Y
> with ",pbe-mt_fhi.UPF" gives total = -200 Ry and when using
> Fe.pbe-sp-van.upf with Y.pbe-nsp-van.upf gives total energy = -1333 Ry.
> Why those two PP give so big difference?
>
> 3. The last one noob-like question:  While doing a 'relax' calc. only
> the X and Y coordinates change during calculation, the Z always is such
> as set on the begining. Is that normal? (or maybe it is only fortuity
> for my atomic system?)
>
> The lecture of pwscf manual did not answer my questions. I regret to
> note that the beginnings with QE are wery hard for novice...
>
> Czestochowa University of Technology, Poland
>
>
>
>
> -- dr inż. Konrad Gruszka Politechnika Częstochowska Instytut Fizyki
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum

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Giuseppe Mattioli
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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ResearcherID: F-6308-2012

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