[Pw_forum] Al pseudopotential and elastic properties

[Jörg Buchwald]

Hi,
does anyone has any experiences with the Al pseudopotentials and the
elastic constants?
I was testing Al.pbe-n-van.UPF with which I could reproduce the elastic
constant and the cohesive energy quite well, but using the output of
the stress matrix I get a way too high results for C11(>150GPa) at a
strain of 0.001. Appliying strains about >3%, I get results which are
quite relastic for C11(90-110 GPa). I thought that this has something
to do whith the kind of approximation, so I also created a ultrasoft
(PZ-)LDA potential with the vanderbilt code and tried the
normconserving Al.pz-vbc.UPF, which gave me similar results.
I applied the strain by stretching the cell vectors and performing a
'relax'-simulation.
This is my input file for a rel. strain of 0.003:
---
 &control
    calculation='relax'
    prefix='Al-test11',
    pseudo_dir='/home/jbuchw/espresso-4.3.2/pseudo'
    outdir = '/home/jbuchw/scratch',
    tstress=.true.
    disk_io='none'
    etot_conv_thr=1.0D-10
    forc_conv_thr=1.0D-10
 /
 &system    
    ibrav=  0, 
    celldm(1) =7.6271186 
    nat=  4, 
    ntyp= 1,
    ecutwfc = 85.0
    occupations='smearing'
    smearing='mp'
    degauss=0.0007
 /
 &electrons
    diagonalization='david'
    mixing_mode='plain'
    conv_thr=1.0D-10
 /
 &ions
   bfgs_ndim=3
 /
 &cell
 !  cell_dofree='yz'
 /
ATOMIC_SPECIES
 Al  26.981539  Al.pbe-n-van.UPF
ATOMIC_POSITIONS crystal
 Al 0.00 0.00 0.00
 Al 0.50 0.50 0.00
 Al 0.00 0.50 0.50
 Al 0.50 0.00 0.50
K_POINTS automatic
 14 14 14 0 0 0
CELL_PARAMETERS
   0.9995541042   0.000000000   0.000000000
   0.000000000   0.996564411   0.000000000
   0.000000000   0.000000000   0.996564411
--
Am I doing something wrong, or does it have something to do with the
pseudopotential?
Regards,
Jörg


PS: I also tried to get C11 by the second derivative the Energy fitting 
quadratic function which gave me C11=75.6GPa but this would assume that
the elastic constant is constant over the strain range(which could be
showed that's not the case for small strains(<0.01) using the
numerical second derivative).



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