[Pw_forum] Convergence issues with Bulk Magnesium

[Giovani Faccin]

Hi Meenakshi,

Perhaps it's not converging because some other parameter's error is larger
than the wfc one. To check it out, you could try this:

ecutwfc=50;  ecutrho=500    (which are not very high neither very low)

and now check the convergence with respect to:
a) degauss value;
b) k-points
c) number of bands (try with bands = 40 once just to be sure).

That said, one thing that is not clear to me: when you built that table,
you have changed the value of ecutwfc. But for each ecutwfc, did you
recalculate the ecutrho so that ecutrho = 8-12 * ecutwfc on each
calculation?

Regards,

Giovani Faccin
UFMS / Brazil


2012/7/5 MMS <mm2422 at cornell.edu>

> Hi,
>
> I am trying to simulate bulk Mg in the hcp structure. Before beginning my
> structural relaxation I wanted to test convergence of the energy values
> with respect to ecutwfc. This is one of the input files.
>
> &control
>         calculation='scf'
>         title='Mgscf'
>         verbosity='high'
>         restart_mode='from_scratch'
>         prefix='Mg'
>         tstress=.true.
>         tprnfor=.true.
>         pseudo_dir='./pseudo/'
>         outdir='./'
> /
> &system
>         ibrav=4
>         celldm(1)=6.0
>         celldm(3)=1.623
>         nat=2
>         ntyp=1
>         nbnd=10
>         ecutwfc=30
>         ecutrho=240
>         occupations='smearing'
>         degauss=0.015
>         smearing='mv'
> /
> &electrons
>         diagonalization='david'
>         mixing_mode='plain'
>         mixing_beta=0.7
>         conv_thr=1.0D-9
> /
> ATOMIC_SPECIES
> Mg 24.305 Mg.pz-n-vbc.UPF
> ATOMIC_POSITIONS {crystal}
> Mg 1/3 2/3 1/4
> Mg 2/3 1/3 3/4
> K_POINTS {automatic}
> 6 6 4 0 0 0
>
> I kept varying ecutwfc, and ecutrho was parametrized to be eight times
> ecutwfc. I had used the perdew-zunger LDA pseudopotential.
>
> Now the ecutwfc values versus the energy values in Rydberg are shown below
> 20 -4.28664244
> 30 -4.28693764
> 40 -4.28699724
> 50 -4.28701233
> 60 -4.28704753
> 70 -4.28717529
> 80 -4.28739243
> 90 -4.28771080
> 100 -4.28803666
> 110 -4.28853887
> 120 -4.28941450
> 130 -4.28990855
> 140 -4.29054764
> 150 -4.29142411
> 160 -4.29309040
>
> What I notice is that the change in energy values (different between
> successive values) initially start decreasing, but slowly start increasing
> and oscillating but always positive, meaning that the energy was
> continually decreasing. This doesnot help me conclude ecutwfc's convergence
> point.
>
> Now, I tried varying the degauss values and even tried increasing the k
> points, but I still see the same behavior. Can I get some pointers in this
> regard?
>
> Meenakshi Sundaram
> Graduate Student
> Mechanical and Aerospace Engg.
> Cornell
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