[Pw_forum] Total energy vs PP and some beginner questions

[Konrad Gruszka]

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...

Thank you in advice for your kindly answer
Konrad Gruszka, PhD.
Czestochowa University of Technology, Poland




-- dr inż. Konrad Gruszka Politechnika Częstochowska Instytut Fizyki