[Pw_forum] Fwd: Hello, why the calculated polarization through Berry Phase is so large

[Nicola Marzari]

First thing to check is the polarization quantum - P is not uniquely
defined, but modulo 2e/omega R, where omega is the volume of the unit 
cell, and R any direct lattice vector. See the work of David Vanderbilt
e.g. Ref 59 of http://www.physics.rutgers.edu/~dhv/pubs/pub_list.html),
or the Rev. Mod. Phys. 1994 of Raffaele Resta.


				nicola


PS: I would exercise some restraint on the significant digits in celldm.


>> From: Xiaofang <lizhanfang at yahoo.com.cn>
>> Date: February 19, 2008 4:00:43  GMT+01:00
>> To: pw_forum at pwscf.org
>> Subject: [Pw_forum] Hello, why the calculated polarization through 
>> Berry Phase is so large
>> Reply-To: PWSCF Forum <pw_forum at pwscf.org>
>>
>> Dear all,
>> I have calculated the polarization of KNbO3 using the Berry Phase. The 
>> atomic position and lattice constants of input are all same with the 
>> experimental values. The computed polarization is 1.1201929 C/m^2,but 
>> the experimental polarization is about 0.37 C/m^2. I even use the 
>> espressov.4.0cvs to calculate, but the result is similar. I have also 
>> tried to calculate the Born effective charge using ph.x, through the 
>> effective charge I get the polarization which is 0.44 C/m^2. I don’t 
>> know why there is so much difference between the two kinds of method 
>> and why the calculated polarization through Berry phase is so large. 
>> Any suggestion is appreciated.
>> This is the Scf’s input
>> &control
>>     calculation  = 'scf'
>>     restart_mode = 'from_scratch'
>>     pseudo_dir   = '/home/pseudo/'
>>     outdir       = '/home/tmp/'
>>  /
>>  &system
>>     ibrav=6
>>     celldm(1)=7.5532353215993782928038366689241
>>     celldm(3)=1.016512384288216162121591193395
>>     nat=5
>>     ntyp=3
>>     nbnd=20
>>     ecutwfc=30.0
>>     ecutrho =300.0
>>     occupations = 'fixed'
>>     degauss=0.00
>>  /
>>  &electrons
>>     diagonalization ='cg'
>>     conv_thr = 1e-12,
>>     mixing_beta=0.3,
>>  /
>> ATOMIC_SPECIES
>>   K    39.0983   019-K-ca-sp-vgrp.uspp.UPF
>>  Nb   92.90638   041-Nb-ca-sp-vgrp.uspp.UPF
>>  O    15.9994    008-O-ca--vgrp.uspp.UPF
>> ATOMIC_POSITIONS (crystal)
>> K        0.5           0.5           0.518
>> Nb       0.000000000   0.000000000   0.0
>> O        0.500000000   0.00000000    0.04
>> O        0.00000000    0.500000000   0.04
>> O        0.00000000    0.00000000    0.544
>>
>> K_POINTS {automatic}
>>    6 6 3 1 1 1
>>
>> This is the BP’s input
>> &control
>>     calculation   = 'nscf'
>>     pseudo_dir    = '/home/pseudo/'
>>     outdir        = '/home/tmp/'
>>     lberry        = .true.
>>     gdir          = 3
>>     nppstr        = 9
>>  /
>> &system
>>     ibrav=6
>>     celldm(1)=7.5532353215993782928038366689241
>>     celldm(3)=1.016512384288216162121591193395
>>     nat=5
>>     ntyp=3
>>     nbnd=20
>>     ecutwfc=30.0
>>     ecutrho =300.0
>>     occupations = 'fixed'
>>     degauss=0.00
>>  /
>>  &electrons
>>     diagonalization ='cg'
>>     conv_thr      = 1e-5
>>     mixing_beta   = 0.3
>>  /
>> ATOMIC_SPECIES
>>   K    39.0983   019-K-ca-sp-vgrp.uspp.UPF
>>  Nb   92.90638   041-Nb-ca-sp-vgrp.uspp.UPF
>>  O    15.9994    008-O-ca--vgrp. uspp.UPF
>> ATOMIC_POSITIONS (crystal)
>> K        0.5           0.5           0.518
>> Nb       0.000000000   0.000000000   0.0
>> O        0.500000000   0.00000000    0.04
>> O        0.00000000    0.500000000   0.04
>> O        0.00000000    0.00000000    0.544
>>
>> K_POINTS {automatic}
>>   6 6 9  1 1 1
>>
>> Thanks in advance.
>> Zhanfang Li
>> Joint training student, Penn State University, USA
>>
>> 雅虎邮箱传递新年祝福，个性贺卡送亲朋！
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> 
> ---
> Paolo Giannozzi, Dept of Physics, University of Udine
> via delle Scienze 208, 33100 Udine, Italy
> Phone +39-0432-558216, fax +39-0432-558222
> 
> 
> 


-- 
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Prof Nicola Marzari   Department of Materials Science and Engineering
13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu