[Pw_forum] convergence of SCF calculation of C6H8

[Axel Kohlmeyer]

On Fri, Nov 2, 2012 at 9:28 PM, Ruibin Liu <rainbee1988 at gmail.com> wrote:
> Thank you! In QE, we use supercell to isolate a molecule, right? I don't

yes, you have no alternative. calculations with
a plane wave basis set always have to be
periodic. for a neutral molecule with no dipole
moment, using a large enough box should work
well enough to decouple the periodic images.

> understand the differences between isolated and periodic systems in QE.
> Anyway, other parameters are as follows:
>  &system
>     ibrav = 1, celldm(1) =30.0, nat= 14, ntyp= 2,

a cubic cell is not exactly the best choice for
a long flat molecule, but with ~15 \AA there
should be sufficient space. i would generally
use a box that is more adjusted to the shape
of the molecule. say a box with 20 x 15 x 10
angstrom instead.

>     ecutwfc = 30.0, ecutrho = 300.0
>  /
>  &electrons
>     diagonalization='david'
>     electron_maxstep = 50,
>     conv_thr = 1.0e-6
>     mixing_beta = 0.1

why don't you go with the defaults
here. they are usually safe. using
a very small mixing beta, like you
do, however, is a bad idea in case
on convergence problems.

be it as it may, i can get an input
based on your info converged in about
10 steps (with the default settings for
the electrons section) and 21 steps
with your choice of parameters. so
there must be something else that
you are doing wrong.

e.g. are the coordinates input correctly?

axel.


> ATOMIC_SPECIES
>  C 12.011   C.blyp-van_ak.UPF
>  H 1.008    H.blyp-van_ak.UPF
> K_POINTS Gamma
>
> How can I change them?
>
> On Fri, Nov 2, 2012 at 2:32 PM, Axel Kohlmeyer <akohlmey at gmail.com> wrote:
>>
>>
>>
>>
>> On Fri, Nov 2, 2012 at 7:22 PM, Ruibin Liu <rainbee1988 at gmail.com> wrote:
>>>
>>> Hi all,
>>>
>>> I tried to do a scf calculation of C6H8 (CH2CHCHCHCHCH2) with the
>>> optimized structure:
>>> 6 -0.331356 1.896930 0.000000
>>> 6 0.315401 3.090807 0.000000
>>> 1 -1.423469 1.885035 0.000000
>>> 1 -0.225850 4.033166 0.000000
>>> 1 1.402796 3.149961 0.000000
>>> 6 0.331356 0.596764 0.000000
>>> 6 -0.331356 -0.596764 0.000000
>>> 1 1.423645 0.599232 0.000000
>>> 6 0.331356 -1.896930 0.000000
>>> 1 -1.423645 -0.599232 0.000000
>>> 6 -0.315401 -3.090807 0.000000
>>> 1 1.423469 -1.885035 0.000000
>>> 1 0.225850 -4.033166 0.000000
>>> 1 -1.402796 -3.149961 0.000000
>>> I have changed the mixing beta several times and also adjusted some
>>> coordinates to break symmetry, but the calculation still cannot converge.
>>> What else should I pay attention to?
>>
>>
>> everything. ;-)
>>
>> there is far to little information (and coordinates don't really help
>> much),
>> to make a qualified statement. if i would do a wild guess, i'd suggest
>> to check if the cell is large enough. atoms might come too close
>> due to PBC. is this supposed to be an isolated or periodic system?
>>
>> axel.
>>
>>
>>>
>>>
>>> Thanks,
>>> Ruibin
>>> --
>>> Liu, Ruibin
>>> Department of Chemistry
>>> Duke University
>>> Durham, NC, 27708
>>>
>>>
>>>
>>> _______________________________________________
>>> Pw_forum mailing list
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>>
>>
>>
>>
>> --
>> Dr. Axel Kohlmeyer  akohlmey at gmail.com  http://goo.gl/1wk0
>> International Centre for Theoretical Physics, Trieste. Italy.
>>
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>
>
>
>
> --
> Liu, Ruibin
> Department of Chemistry
> Duke University
> Durham, NC, 27708
>
>
>
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-- 
Dr. Axel Kohlmeyer  akohlmey at gmail.com  http://goo.gl/1wk0
International Centre for Theoretical Physics, Trieste. Italy.