[QE-users] vc-relax

[Sai Pavan Chitta]

Dear Dr Baroni,

Thank you very much for your insightful answer. Though, I am still confused
about the k-points sampling. This is what I thought about the sampling. In
a calculation concerning a single monolayer, we do not need to look at many
k-points in the BZ as no atoms exist in that direction. Therefore, the
k-space can be represented by a single k-point in the z-dimension.

But, as I start to make the calculations for a bi-layer, it is now
imperative for me to use more number of k-points(to make sure that the
calculation converges) in the z-direction as there are more than one atoms
in that dimension.

This logic has also helped me to work with skewed and non-planar monolayer
lattices. Please let me know if my explanation to the logic makes sense in
this case.


Thanks and Regards
Chitta Sai Pavan
IIT Gandhinagar


On Tue, 9 Oct 2018, 11:39 Stefano Baroni, <baroni at sissa.it> wrote:

> Sorry, Chitta, but this does not make much sense. k-point sampling is
> intimately related to periodicity: no periodicity, No k-point sampling. The
> dependence of any quantity of a non periodic system on the number of k
> point simply shows that the size of the supercell may be too small. Using
> k-point sampling may in this case speed up convergence towards the
> infinite-size limit, but in doing so one should know what one is doing and
> why, which this discussion shows not to be necessarily the case. The rule
> of thumb is: if your result depend too much on the number of k point,
> increase the size of the cell until it does not. The number of monolayers
> has nothing to do with the rate of convergence. The vacuum in between pairs
> of monolayers does. Regards — Stefano B
>
>
> On 9 Oct 2018, at 06:34, Sai Pavan Chitta <chitta.sai at iitgn.ac.in> wrote:
>
> Dear Sabike,
>
> In my experience of working with monolayers, I found that the number of
> k-points in the k-mesh in z direction aalso affects the convergence. The
> energy starts converging the moment I increase the number of k-points in
> z-direction. It also makes sense as you are using two monolayers in your
> model and thus requires more k-points in the z-direction.
>
> Moreover, you can make the convergence threshold more stringent to about
> 1e-8 for a proper convergence.
>
> Regards
> Chitta Sai Pavan
> IIT Gandhinagar
>
>
> On Tue, 9 Oct 2018, 08:18 Sabike Ghasemi, <s.ghasemi at du.ac.ir> wrote:
>
>> dear all
>> i want to relax atomic coordinates and lattic constants for graphene
>> bilayer. but it dosen't converge.
>> input file is:
>>
>>
>>  &CONTROL
>>                        title = GRAPHENEBILAYER ,
>>                  calculation = 'vc-relax' ,
>>                 restart_mode = 'from_scratch' ,
>>                   wf_collect = .true. ,
>>                       outdir = './out' ,
>>                       wfcdir = './out' ,
>>                   pseudo_dir = './' ,
>>                       prefix = 'GBI' ,
>>                    verbosity = 'high' ,
>>                etot_conv_thr = 1.0D-4 ,
>>                forc_conv_thr = 1.0D-4 ,
>>                      tstress = .true. ,
>>                      tprnfor = .true. ,
>>  /
>>  &SYSTEM
>>                        ibrav = 4,
>>                    celldm(1) = 4.648726,
>>                    celldm(3) = 9.512195384,
>>                          nat = 4,
>>                         ntyp = 1,
>>                      ecutwfc = 50 ,
>>                      ecutrho = 500 ,
>>                        nosym = .false. ,
>>                  occupations = 'smearing' ,
>>                      degauss = 0.002 ,
>>                     smearing = 'gaussian' ,
>>  /
>>  &ELECTRONS
>>             electron_maxstep = 200,
>>                     conv_thr = 1.0D-6 ,
>>                  mixing_beta = 0.3 ,
>>              diagonalization = 'david' ,
>>  /
>>  &IONS
>>                 ion_dynamics = 'bfgs' ,
>>  /
>>  &CELL
>>                cell_dynamics = 'bfgs' ,
>>                  cell_factor = 2.D0 ,
>>                  cell_dofree = '2Dxy' ,
>>  /
>> ATOMIC_SPECIES
>>     C   12.00000  C.blyp-mt.UPF
>> ATOMIC_POSITIONS crystal
>>     C      0.333333330    0.666666660    0.572649573
>>     C      1.000000000    0.000000000    0.572649753
>>     C      0.666666660    0.333333330    0.427350427
>>     C      1.000000000    0.000000000    0.427350427
>> K_POINTS automatic
>>   16 16 1   0 0 0
>>
>>
>> best,
>> sabike
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> —
> Stefano Baroni - Trieste —  http://stefano.baroni.me
>
>
>
>
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