[Pw_forum] Phonons in 2D boron nitride

[Francesco Delodovici]

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,
/
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