[Pw_forum] forces in the 'sawtooth' method

[Liping Yu]

Just one comment on this:

The Born effective charge should be defined as Force/macroscopic internal
field. The applied field in the slab by sawtooth method actually is the
electric displacement D not the field in the slab. So your obtained
effective charge maybe differ from the one from bulk phonon calculation by
 a factor of the dielectric constant of the slab. On the other hand,
please note that for such a thin slab, the calculated dielectric constant
may depend on how you average the field or polarization of the slab, and
may differ a lot from bulk dielectric constant.

Best

Liping YU
Department of Physics,
North Carolina State University

> Dear PWSCF users,
>    In order to check whether the forces in the 'sawtooth' method are
> calculated correctly in the latest version-4.0.2(no offense intended), we
> carry out the following test. We calculate an isolated symmetric 5-layer
> STO
> slab (input file is attached below). In the absence of electric field,
> there
> are no forces on the middle TiO2 layer due to symmetry. Then we turn on a
> weak electric field (0.0001 a.u.) and obtain the forces on Ti and on O are
> 0.00046 and -0.00013 Ry/au, respectively (in the middle TiO2 layer).
> Therefore the Born effective charges of Ti and O are approximately
>
> e*(Ti)=0.00046/0.0001/2= 2.3
> e*(O)=-0.00013/0.0001/2= -0.65
>
> The factor '2' is due to the units. However from both phonon frequency
> calculation and literature, the Born effective charges of Ti and O are:
>
> e*(Ti)= 7.25
> e*(O)= -2.06
>
> I am just wondering whether my method is correct. If it is correct, does
> that imply there might be something wrong with the forces in the
> 'sawtooth'
> method.
> Thank you very much.
>
> Hanghui Chen
> Department of Physics, Yale University
>
> &CONTROL
>   calculation='scf'
>   wf_collect=.true.
>   pseudo_dir = './psp'
>   outdir='.'
>   wfcdir='/scratch'
>   prefix='STO-5.5uc-EF0.0001-scf'
>   tprnfor = .true.
>   tstress = .true.
>   disk_io='low'
>   verbosity='high'
>   dt=80.D0
>   tefield=.true.
> /
> &SYSTEM
>   ibrav=  6
>   celldm(1) = 7.27
>   celldm(3) = 10.0
>   nat=  27
>   ntyp= 3
>   ecutwfc = 30.0
>   ecutrho = 180.0
>   occupations='smearing'
>   smearing=gauss
>   degauss=0.005
>   edir=3
>   emaxpos=0.8
>   eopreg=0.1
>   eamp=0.0001
> /
> &ELECTRONS
>   diagonalization='david'
>   mixing_beta = 0.7D0
>   diago_david_ndim = 4
>   startingwfc='atomic'
>   startingpot='file'
>   conv_thr = 1.0d-8
> /
> &IONS
>   ion_dynamics = 'damp'
>   phase_space = 'full'
>   pot_extrapolation = 'second_order'
>   wfc_extrapolation = 'second_order'
> /
> &CELL
>   cell_dynamics = 'damp-w'
> /
> ATOMIC_SPECIES
>  Sr  87.62   038-Sr-ca-sp-vgrp.uspp.format.UPF
>  Ti  47.90   022-Ti-ca-sp-vgrp.uspp.format.UPF
>  O   16.00   008-O-ca--vgrp.uspp.format.UPF
> ATOMIC_POSITIONS alat
>  Sr 0.00 0.00 0.00
>  O  0.50 0.50 0.00
>  Ti 0.50 0.50 0.50
>  O  0.00 0.50 0.50
>  O  0.50 0.00 0.50
>  Sr 0.00 0.00 1.00
>  O  0.50 0.50 1.00
>  Ti 0.50 0.50 1.50
>  O  0.00 0.50 1.50
>  O  0.50 0.00 1.50
>  Sr 0.00 0.00 2.00
>  O  0.50 0.50 2.00
>  Ti 0.50 0.50 2.50
>  O  0.00 0.50 2.50
>  O  0.50 0.00 2.50
>  Sr 0.00 0.00 3.00
>  O  0.50 0.50 3.00
>  Ti 0.50 0.50 3.50
>  O  0.00 0.50 3.50
>  O  0.50 0.00 3.50
>  Sr 0.00 0.00 4.00
>  O  0.50 0.50 4.00
>  Ti 0.50 0.50 4.50
>  O  0.00 0.50 4.50
>  O  0.50 0.00 4.50
>  Sr 0.00 0.00 5.00
>  O  0.50 0.50 5.00
> K_POINTS {automatic}
> 8 8 1 0 0 0
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum
>