Re: [QE-users] vc-relax

[Giuseppe Mattioli]

Dear Sabike
I see that you are using the BLYP functional, and I suppose that this  
is what you want to use. But 50 Ry for a norm-conserving  
Martins-Troullier C pseudopotential is probably a poor cutoff that can  
affect convergence. Moreover, there is no need to specify ecutrho for  
NC pseudopotentials, as it should be automatically fixed at 4*ecutwfc  
(check your output file). Finally, there are also a few parameters  
that can be adjusted to improve convergence, but it would be useful to  
know what you mean when you say "it doesn't converge". Does your  
calculation reach the maximum electron_maxstep or nstep?
HTH
Giuseppe
Stefano Baroni <bar...@sissa.it> ha scritto:

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....@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.ghas...@du.ac.ir  
<mailto:s.ghas...@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/>



GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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I-00015 - Monterotondo Scalo (RM)
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E-mail: <giuseppe.matti...@ism.cnr.it>

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