[Pw_forum] (no subject)

[Axel Kohlmeyer]

On Tue, Jun 18, 2013 at 6:57 PM, shiva mokhavat
<shiva.mokhavat at gmail.com> wrote:
> Dear users
> I have a question about the output of vc-relax. when I run vc-relax for
> orthorhombic structure, the output celldms are so different from
> experimental data. forexample celldm(1) is 8.896 bohr from experimental
> data, but the output celldm(1) from vc-relax is 6.960 bohr.

this means, that either your calculation is not correct (bad input
data, not fully converged cutoff, kpoints&smearing, wrong geometry),
or the material is not well represented by pseudopotential DFT calculations.

computer programs have the sometimes irritating property, that they
blindly follow what we feed them and compute whatever comes out of
that. there isn't yet a "oh *this* is what you wanted me to do" unit
in modern CPUs. ;-)

axel.


> my input file is:
> &CONTROL
>       calculation='vc-relax'
>      restart_mode ='from_scratch',
>            outdir = '/home/izadi/tmp/vc.orthorhombic',
>            prefix = 'VC',
>        pseudo_dir = '/home/izadi/pseudo/',
>          tstress  = .TRUE.,
>          tprnfor  = .TRUE.,
>  /
>  &SYSTEM
>         ibrav = 9,
>     celldm(1) = 8.896,
>     celldm(2) = 1.051,
>     celldm(3) = 0.998,
>            nat= 8,
>          ntyp = 2,
>         nspin = 1,
>       ecutwfc = 30,
>       ecutrho = 120,
>   occupations = 'smearing',
>        degauss= 0.01,
> &electrons
>      conv_thr=1.D-5,
>      mixing_beta=0.7,
>  /
>
> &ELECTRONS
>          conv_thr = 1.D-6,
>       mixing_beta = 0.7,
> /
>  &IONS
>   ion_dynamics = 'damp'
> /
>  &CELL
>  cell_dynamics='damp-w'
>  /
> ATOMIC_SPECIES
>  Ga    69.72   Ga.pz-bhs.UPF
>   P     30.974  P.pz-bhs.UPF
>
> ATOMIC_POSITIONS   (crystal)
>  Ga     0.000     0.624    0.250
>  Ga     0.000     0.376    0.750
>  Ga     0.500     0.124    0.250
>  Ga     0.500     0.876    07500
>  P      0.000     0.152    0.250
>  P      0.000     0.848    0.750
>  P      0.500     0.652    0.250
>  P      0.500     0.348    0.750
> K_POINTS   (automatic)
> 4 3 5    0  0  0
>
>
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--
Dr. Axel Kohlmeyer  akohlmey at gmail.com  http://goo.gl/1wk0
International Centre for Theoretical Physics, Trieste. Italy.