[QE-users] vc-relax

[Stefano Baroni]

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 <mailto: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 <http://stefano.baroni.me/> 




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