<div dir="ltr"><div><div><div>Dear Francesco,<br><br></div>As far as I understood, smearing is probably creating you a problem. This is preferably used when you deal with metallic system. Try with "occupation = fixed". The other thing could be due to the ecutwfc. You need to check this cut off with respect to each element and take the lowest converged one for quick results. This also depends on the type of pseudo you are using. Relaxation with 20 20 1 is good, but so large sampling is not required as it is easy to converge the phonon frequencies. 12 12 1 or 14 14 1 scf will do the job, even in my case 6 6 1 does this. You may also use this to generate less number of regular q-points along the BZ route. Specifically do also check with any double prime frequencies (like A" or so) in your report file. These are bad for single layers.<br></div><div>Hope this will help you..</div><div><br></div>Regards,<br></div>Sitangshu <br></div><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Dec 12, 2017 at 3:51 AM, Francesco Delodovici <span dir="ltr"><<a href="mailto:francesco.delodovici@unimi.it" target="_blank">francesco.delodovici@unimi.it</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">Hello all!</span><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">I have some troubles with phonons in two dimensional hexagonal Boron Nitride, and I am looking for some hints: </div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">1) the out-of-plane acoustic branch at low wave vector is not quadratic, istead is linear. </div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">At a first glance, I thought this could be related to some residual forces, but I performed two successive vc-relax calculations</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">and they are 0.000000 according to the output. There is some residual pressure on z axis, but I suppouse it is related to the constrain</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">on the c primitive vector (through the flag cell_dofree = '2Dxy' ). </div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">I doubt that this is due to some periodic images interaction, because the cell is 20 A in the direction perpendicular to the layer plane.</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">I also increased the q-point mesh from 6x6x1 up to 10x10x1 in the ph.x input. In the latter case the out-of-plane acoustic branch turns out </div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">to have a parabolic-like behaviour around Gamma but with opposite concavity.</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">2) Since I don't include the flag epsil='true' there is no LO-TO splitting, but the slope of the LO branch is vanishing at Gamma</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">and it shouldn't. Any chance that some option exists to obtain correct behaviour of otpical phonons in 2D for vanishing wave vector? </div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">I'm using quantum espersso v6.0.</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">Find attached below the input file for vc-relax and phonons. </div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">Any suggestion will be appreciated,</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">thank you!</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">Francesco Delodovici, Phd Student</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">Università degli studi di Milano</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">############ VC RELAX ###################</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px"> &CONTROL</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px"><div> title = 'hbn' ,</div><div> calculation = 'vc-relax' ,</div><div> restart_mode = 'from_scratch' ,</div><div> outdir = '/localdisk1/delodovici/' ,</div><div> pseudo_dir = '~/Pseudop/' ,</div><div> prefix = 'hbn' ,</div><div> forc_conv_thr = 1D-8 ,</div><div> etot_conv_thr = 1D-8 ,</div><div> wf_collect = .false. ,</div><div> /</div><div> &SYSTEM</div><div> ibrav = 4 ,</div><div><span style="white-space:pre-wrap"> </span> celldm(1) = 4.695926777902829 ,</div><div><span style="white-space:pre-wrap"> </span> celldm(3) = 8.048362859199854 ,</div><div> nat = 2 ,</div><div> ntyp = 2 ,</div><div> ecutwfc = 50 ,</div><div> ecutrho = 200 ,</div><div> occupations = 'smearing' ,</div><div><span style="white-space:pre-wrap"> </span> smearing = 'gaussian' ,</div><div> degauss = 2e-4 ,</div><div> nspin = 1 ,</div><div>/</div><div> &ELECTRONS</div><div> conv_thr = 1.0d-9</div><div> /</div><div> &IONS</div><div> ion_dynamics = 'bfgs'</div><div> /</div><div> &CELL</div><div> cell_dynamics = 'bfgs'</div><div> cell_dofree = '2Dxy'</div><div> /</div><div>ATOMIC_SPECIES</div><div>N 14.007 N.pz-rrkjus.UPF</div><div>B 10.810 B.pz-n-rrkjus_psl.0.1.UPF </div><div><br></div><div>ATOMIC_POSITIONS crystal</div><div>B 0.000000 0.000000 0.000000</div><div>N 0.333333 0.666666 0.000000</div><div><br></div><div>K_POINTS automatic</div><div>20 20 1 0 0 0</div></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">##############################<wbr>##################</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px">############### PHONONS ##################</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12.8px"><div><br></div><div>phonons of hBN</div><div> &inputph</div><div> tr2_ph=1.0d-14,</div><div> prefix='hbn',</div><div> amass(1)=14.007,</div><div> amass(2)=10.810,</div><div> outdir='/localdisk1/delodovici<wbr>',</div><div> fildyn='phonons.dyn',</div><div> ldisp=.true.,</div><div> nq1=10,</div><div> nq2=10,</div><div> nq3=1,</div><div>/</div></div>
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