[Pw_forum] Phonons in 2D boron nitride

[Sitangshu Bhattacharya]

Dear Francesco,

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.
Hope this will help you..

Regards,
Sitangshu

On Tue, Dec 12, 2017 at 3:51 AM, Francesco Delodovici <
francesco.delodovici at unimi.it> wrote:

> 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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-- 
**********************************************
Sitangshu Bhattacharya (সিতাংশু ভট্টাচার্য), Ph.D
Assistant Professor,
Room No. 2221, CC-1,
Nanoscale Electro-Thermal Laboratory,
Department of Electrical and Communication Engineering,
Indian Institute of Information Technology-Allahabad
Uttar Pradesh 211 012
India
Telephone: 91-532-2922000 Extn.: 2131
Web-page: http://profile.iiita.ac.in/sitangshu/
Institute: http://www.iiita.ac.in/
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