[Pw_forum] Zn NCPP pseudopotential

[giacsport at libero.it]

Dear All,
             excuse me if the question maybe can appear as "weird" but I am 
quite new with PWSCF.

I would like to do some calculations on ZnO systems by using NCPP (in 
principle PBE at this level could be OK)

Anyway, I checked the database of pseudopotentials and I noticed that an NCPP 
for Zn is missing.
Reading the suggestions in the Doc  I went to the Rappe Group PSP Page for 
GGA 
generated Potentials and I found Zn psp in abinit (fhi) format.
( http://www.sas.upenn.edu/rappegroup/htdocs/Research/PSP/zn.optgga1.fhi )

I used the "fhi2upf" tools  following the instructions required and thus I 
generated both a NCPP for both Zn and O.
(In details     Zn  4S,2 4P,0 3d,10 3f,0      and     O 2S,2  2P,4)
then I tested it on a "vc-relax' calculation on the WZ-like structure of ZnO


  this is my input file

>&control
>  calculation='vc-relax',
>  pseudo_dir='/home/giacomo/src/pseudo/'
>  prefix='ZnO'
>  wf_collect=.true.
>  outdir='/scr/giacomo/PWSCF_test/ZnO/test_pseudo/'
>  tprnfor=.true.
>  etot_conv_thr=1.0D-8
>  forc_conv_thr=1.0D-4
>/
>&system
> ntyp=2, nat=4, ibrav=4, a=3.2417, c= 5.18760,
> ecutwfc= 30., force_symmorphic=.true.
>/
>&ELECTRONS
>    diagonalization='david'
>    mixing_mode = 'plain'
>    mixing_beta = 0.7
>    conv_thr =  1.0d-8
>/
>&IONS
>  ion_dynamics = 'bfgs',
>/
>&cell
>        cell_dynamics='bfgs'
>/
>ATOMIC_SPECIES
>Zn 65.39   30-Zn.PBE.fhi.UPF
>O  15.9994  08-O.PBE.fhi.UPF
>ATOMIC_POSITIONS  crystal
>  Zn    0.33330   0.66670   0.00000
>  Zn    0.66660   0.33330   0.50000
>  O     0.33330   0.66670   0.38190
>  O     0.66660   0.33330   0.88190
>K_POINTS (automatic)
> 4 4 4 1 1 1



and this is what I got in the output:

.....
>     bravais-lattice index     =            4
>     lattice parameter (a_0)   =       6.1259  a.u.
>     unit-cell volume          =     318.5953 (a.u.)^3
>     number of atoms/cell      =            4
>     number of atomic types    =            2
>     number of electrons       =        36.00
>     number of Kohn-Sham states=           18
>     kinetic-energy cutoff     =      30.0000  Ry
>     charge density cutoff     =     120.0000  Ry
>     convergence threshold     =      1.0E-08
>     mixing beta               =       0.7000
>     number of iterations used =            8  plain     mixing
>     Exchange-correlation      = SLA  PZ   NOGX NOGC (1100)
>     nstep                     =           50
>
>
>     celldm(1)=   6.125925  celldm(2)=   0.000000  celldm(3)=   1.600271
>     celldm(4)=   0.000000  celldm(5)=   0.000000  celldm(6)=   0.000000
>
>     crystal axes: (cart. coord. in units of a_0)
>               a(1) = (  1.000000  0.000000  0.000000 )
>               a(2) = ( -0.500000  0.866025  0.000000 )
>               a(3) = (  0.000000  0.000000  1.600271 )
>....
>     Initial potential from superposition of free atoms
>     Check: negative starting charge=   -0.000653
>
>     starting charge   35.99993, renormalised to   36.00000
>.....


and at the end

>.....
>     bfgs converged in  13 scf cycles and  11 bfgs steps
>     (criteria: energy < 0.10E-07, force < 0.10E-03, cell < 0.50E+00)
>
>     End of BFGS Geometry Optimization
>
>     Final enthalpy =    -269.5939008350 Ry
>     new unit-cell volume =    196.26255 a.u.^3 (    29.08311 Ang^3 )
>
>CELL_PARAMETERS (alat)
>   0.850885542  -0.000000001   0.000009624
>  -0.425442771   0.736893797  -0.000004810
>   0.000015404   0.000000004   1.361589938
>
>ATOMIC_POSITIONS (crystal)
>Zn       0.333283929   0.666666020   0.000176085
>Zn       0.666617909   0.333333981   0.500176085
>O        0.333282225   0.666666288   0.381723915
>O        0.666615937   0.333333711   0.881723915
>
>......

that makes me very suspicious about a so big reduction of the unit-cell 
volume.
I know that there are a lot of variables that can influence this result (not 
only the pseudopotential), but since I am a dummy in creating them I wonder 
if 
someone can give me few hints in order to improve my ones. or even better if 
someone could share an already tested  NCPP at PBE level I would really 
appreciate it. 

Thanks in advance

Very best,
Giacomo


Giacomo Giorgi, Ph. D.,
Department of Chemical System Engineering,
School of Engineering, The University of Tokyo,
Japan