Hello all! I have some troubles with phonons in two dimensional hexagonal Boron Nitride, and I am looking for some hints:
1) the out-of-plane acoustic branch at low wave vector is not quadratic, istead is linear. At a first glance, I thought this could be related to some residual forces, but I performed two successive vc-relax calculations 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 on the c primitive vector (through the flag cell_dofree = '2Dxy' ). 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. 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 to have a parabolic-like behaviour around Gamma but with opposite concavity. 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 and it shouldn't. Any chance that some option exists to obtain correct behaviour of otpical phonons in 2D for vanishing wave vector? I'm using quantum espersso v6.0. Find attached below the input file for vc-relax and phonons. Any suggestion will be appreciated, thank you! Francesco Delodovici, Phd Student Università degli studi di Milano ############ VC RELAX ################### &CONTROL title = 'hbn' , calculation = 'vc-relax' , restart_mode = 'from_scratch' , outdir = '/localdisk1/delodovici/' , pseudo_dir = '~/Pseudop/' , prefix = 'hbn' , forc_conv_thr = 1D-8 , etot_conv_thr = 1D-8 , wf_collect = .false. , / &SYSTEM ibrav = 4 , celldm(1) = 4.695926777902829 , celldm(3) = 8.048362859199854 , nat = 2 , ntyp = 2 , ecutwfc = 50 , ecutrho = 200 , occupations = 'smearing' , smearing = 'gaussian' , degauss = 2e-4 , nspin = 1 , / &ELECTRONS conv_thr = 1.0d-9 / &IONS ion_dynamics = 'bfgs' / &CELL cell_dynamics = 'bfgs' cell_dofree = '2Dxy' / ATOMIC_SPECIES N 14.007 N.pz-rrkjus.UPF B 10.810 B.pz-n-rrkjus_psl.0.1.UPF ATOMIC_POSITIONS crystal B 0.000000 0.000000 0.000000 N 0.333333 0.666666 0.000000 K_POINTS automatic 20 20 1 0 0 0 ################################################ ############### PHONONS ################## phonons of hBN &inputph tr2_ph=1.0d-14, prefix='hbn', amass(1)=14.007, amass(2)=10.810, outdir='/localdisk1/delodovici', fildyn='phonons.dyn', ldisp=.true., nq1=10, nq2=10, nq3=1, / <signaturebeforequotedtext></signaturebeforequotedtext><signatureafterquotedtext></signatureafterquotedtext>
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