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, <[email protected]> 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 <[email protected]> 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, <[email protected]> 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 >> _______________________________________________ >> users mailing list >> [email protected] >> https://lists.quantum-espresso.org/mailman/listinfo/users > > _______________________________________________ > users mailing list > [email protected] > https://lists.quantum-espresso.org/mailman/listinfo/users > > > — > Stefano Baroni - Trieste — http://stefano.baroni.me > > > > > _______________________________________________ > users mailing list > [email protected] > https://lists.quantum-espresso.org/mailman/listinfo/users
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