[Pw_forum] forces in the 'sawtooth' method

[alan chen]

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
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