[Pw_forum] convergence for lattice parameter

[Giovani Faccin]

Dear Ayedun,

Try using a higher ecutwfc, like 50 for example. Also a smaller degauss
value. Finally set the ecutrho to 4*ecutwfc if your pseudopotential is not
US, and to 12*ecutwfc if it is.

It`s also a good idea to specify the number of bands, to make sure you got
enough ones on the calculation. If unsure you can just use a high value and
reduce it after a test calculation to compensate for the extra empty bands.

Choose a single ibrav value (if unsure, go for the experimental value or
close to it) and play with these parameters, checking how the total energy
changes with them. Once you get convergence on the total energy, then
you'll have a reliable setup that will allow you to change the ibrav values
and obtain good curves.

Sincerely,

Giovani Faccin

UFMS/Brazil


2012/4/8 funmi ayedun <funmiayedun at yahoo.com>

> Dear All,
>
> Good day all, I am a new user in quantum espresso, I am working on anatase
> TiO2 (with hexagonal structure) but  I have problem with convergence test
> for lattice parameter for TiO2 system.  I have tried to vary celldm 1 from
> 8.2 to 9.0Bohr but I was unable get a convergence result. Also, I tried to
> vary celldm 3  from 0.2441 to 1.3441Bohr and  keeping celldm 1 constant, I
> was unable to get convergence as well. I have my input file below.
>
> &control
>   calculation = 'scf',
>   restart_mode='from_scratch',
>   prefix='TiO2'
>   pseudo_dir='/home/funmilayo/espresso-4.2.1/pseudo',
>   outdir='./',
>     tprnfor = .true.
>   tstress = .true.
> /
> &system
>   ibrav=6, celldm(1)= 8.6808, celldm(3)= 0.7450,
>   nat=3, ntyp=2,occupations='smearing', smearing='marzari-vanderbilt',
> degauss=0.02
>   ecutwfc= 25.0,
> /
> &electrons
>  mixing_beta = 0.7
>  diagonalization='davidson'
> conv_thr =  1.0d-8
> /
> ATOMIC_SPECIES
> Ti 47.867  Ti.blyp-sp-van_ak.UPF
> O  15.994  O.blyp-van_ak.UPF
> ATOMIC_POSITIONS
> Ti 0.00000000   0.00000000   0.00000000
> O  0.33333333   0.33333333   0.33333333
> O  0.80000000   0.20000000   0.50000000
> K_POINTS (automatic)
> 6 6 6  0  0  0
>
> Can anyone please tell me where I have gone wrong.
> Thank you all.
>
>
> Ayedun Oluwafunmilayo,
> PhD student,
> Department of Physics,
> University of Agriculture,
> Abeokuta,
> Nigeria.
>
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>
>
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