[Pw_forum] problems with pressure calculation

[Eduardo Ariel Menendez Proupin]

Dear colleagues,

I have found strange differences in results for the calculation of pressure
using pwscf in versions 4.3 and 5.0.N (N=1,2).  We are calculating wurtzite
CdSe, using the following UPF files.

PAW calculation
   Cd  112.40000  Cd.pbe-dn-kjpaw_psl.0.2.UPF
   Se   78.96000  Se.pbe-n-kjpaw_psl.0.2.UPF


Ultrasoft PP calculation
   Cd  112.40000  Cd.pbe-dn-rrkjus_psl.0.2.UPF
   Se   78.96000  Se.pbe-n-rrkjus_psl.0.2.UPF



For the PAW calculations, the convergence of pressure with the density
cutoff (euctrho) shows jumps  (see the attached  figure paw-press.eps) and
no convergence in the case of version 5.0.N, while there is convergence
with version 4.3. For both versions the total energies are the same.
The jumps are associated with differences in the number of symmetry
operations detected. The message

 "         (note:  6 additional sym.ops. were found but ignored
           their fractional translations are incommensurate with FFT grid)"
is associated with higher pressures (33 kbar). Lower pressures ~ 26 kbar)
are found when the FFT grid is commensurate and all 12 sym. ops. are used.
In the calculations with version 4.3 the pressures are somewhat lower, but
apparently convergent. Also, there are less symmetry operations detected or
used than for version 5.0.N.

In the ultrasoft calculations, (see figure uspp-press), both versions 4.3
and 5.0.N produce convergent pressures vs ecutrho, but the pressures differ
almost by 10 kbar. (The total energies are almost equal, as expected).

Thanks for any comment.

-- 
Eduardo Menendez Proupin
Departamento de Fisica, Facultad de Ciencias, Universidad de Chile
URL: http://fisica.ciencias.uchile.cl/~emenendez
*"Lograremos el anhelado desarrollo cuando sea la ciudadanía, y no los
científicos, la que pida más ciencia para Chile." J. Babul*
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