<div dir="auto"><div>Dear Dr Baroni,</div><div dir="auto"><br></div><div dir="auto">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.</div><div dir="auto"><br></div><div dir="auto">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.</div><div dir="auto"><br></div><div dir="auto">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. <br><div dir="auto"><br></div><div dir="auto"><br>Thanks and Regards</div><div dir="auto"><div data-smartmail="gmail_signature" dir="auto">Chitta Sai Pavan<br>IIT Gandhinagar</div></div><br><br><div class="gmail_quote" dir="auto"><div dir="ltr">On Tue, 9 Oct 2018, 11:39 Stefano Baroni, <<a href="mailto:baroni@sissa.it">baroni@sissa.it</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word;line-break:after-white-space">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<div><br><div><br><blockquote type="cite"><div>On 9 Oct 2018, at 06:34, Sai Pavan Chitta <<a href="mailto:chitta.sai@iitgn.ac.in" target="_blank" rel="noreferrer">chitta.sai@iitgn.ac.in</a>> wrote:</div><br class="m_-3956190392573869776Apple-interchange-newline"><div><div dir="auto"><div>Dear Sabike,</div><div dir="auto"><br></div><div dir="auto"><div dir="auto">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. </div><div dir="auto"><br></div><div dir="auto">Moreover, you can make the convergence threshold more stringent to about 1e-8 for a proper convergence.</div><div dir="auto"><br></div><div dir="auto">Regards</div><div dir="auto"><div data-smartmail="gmail_signature" dir="auto">Chitta Sai Pavan<br>IIT Gandhinagar</div></div><br><br><div class="gmail_quote" dir="auto"><div dir="ltr">On Tue, 9 Oct 2018, 08:18 Sabike Ghasemi, <<a href="mailto:s.ghasemi@du.ac.ir" target="_blank" rel="noreferrer">s.ghasemi@du.ac.ir</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div style="font-family:arial,helvetica,sans-serif;font-size:12pt"><div>dear all<br></div><div>i want to relax atomic coordinates and lattic constants for graphene bilayer. but it dosen't converge.<br></div><div>input file is:<br></div><div><br></div><div><br></div><div> &CONTROL<br> title = GRAPHENEBILAYER ,<br> calculation = 'vc-relax' ,<br> restart_mode = 'from_scratch' ,<br> wf_collect = .true. ,<br> outdir = './out' ,<br> wfcdir = './out' ,<br> pseudo_dir = './' ,<br> prefix = 'GBI' ,<br> verbosity = 'high' ,<br> etot_conv_thr = 1.0D-4 ,<br> forc_conv_thr = 1.0D-4 ,<br> tstress = .true. ,<br> tprnfor = .true. ,<br> /<br> &SYSTEM<br> ibrav = 4,<br> celldm(1) = 4.648726,<br> celldm(3) = 9.512195384,<br> nat = 4,<br> ntyp = 1,<br> ecutwfc = 50 ,<br> ecutrho = 500 ,<br> nosym = .false. ,<br> occupations = 'smearing' ,<br> degauss = 0.002 ,<br> smearing = 'gaussian' ,<br> /<br> &ELECTRONS<br> electron_maxstep = 200,<br> conv_thr = 1.0D-6 ,<br> mixing_beta = 0.3 ,<br> diagonalization = 'david' ,<br> /<br> &IONS<br> ion_dynamics = 'bfgs' ,<br> /<br> &CELL<br> cell_dynamics = 'bfgs' ,<br> cell_factor = 2.D0 ,<br> cell_dofree = '2Dxy' ,<br> /<br>ATOMIC_SPECIES<br> C 12.00000 C.blyp-mt.UPF <br>ATOMIC_POSITIONS crystal <br> C 0.333333330 0.666666660 0.572649573 <br> C 1.000000000 0.000000000 0.572649753 <br> C 0.666666660 0.333333330 0.427350427 <br> C 1.000000000 0.000000000 0.427350427 <br>K_POINTS automatic <br> 16 16 1 0 0 0 <br><br></div><div><br></div><div>best, <br></div><div>sabike<br></div></div></div>_______________________________________________<br>
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<span class="m_-3956190392573869776Apple-style-span" style="border-collapse:separate;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-alternates:normal;font-variant-east-asian:normal;line-height:normal;text-indent:0px;border-spacing:0px"><div style="word-wrap:break-word"><div><div><span class="m_-3956190392573869776Apple-style-span" style="border-collapse:separate;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-alternates:normal;font-variant-east-asian:normal;line-height:normal;text-indent:0px;border-spacing:0px"><span class="m_-3956190392573869776Apple-style-span" style="border-collapse:separate;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-alternates:normal;font-variant-east-asian:normal;line-height:normal;text-indent:0px;border-spacing:0px"><div style="word-wrap:break-word"><div><span style="font-size:12px">— </span></div><div style="word-wrap:break-word"><div style="word-wrap:break-word"><div style="word-wrap:break-word"><div style="word-wrap:break-word"><div style="word-wrap:break-word"><div><div style="margin:0px"><font class="m_-3956190392573869776Apple-style-span"><span style="font-size:12px">Stefano Baroni - Trieste — </span><a href="http://stefano.baroni.me" style="color:rgb(0,0,0);font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-transform:none;white-space:normal;word-spacing:0px" target="_blank" rel="noreferrer">http://stefano.baroni.me</a><span style="font-size:12px"> </span></font></div></div><div style="color:rgb(0,0,0);font-family:Helvetica;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-transform:none;white-space:normal;word-spacing:0px"><br></div></div></div></div></div></div></div></span></span></div></div></div></span><br class="m_-3956190392573869776Apple-interchange-newline"><br class="m_-3956190392573869776Apple-interchange-newline">
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