[Pw_forum] Phonons in 2D boron nitride

[Sitangshu Bhattacharya]

Sorry: Highest (not lowest converged...)

On Tue, Dec 12, 2017 at 10:43 AM, Sitangshu Bhattacharya <
sitangshu at iiita.ac.in> wrote:

> 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/
>
>


-- 
**********************************************
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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