[Pw_forum] the sum of the born effective charge in phonon calculation

[程迎春]

Dear pwscf users,
    I calculated the phonon of ZnS at Gamma point.  I checked the output
file, and found that the sum of the born effective charge is not zero.
================================
          Effective charges E-U in cartesian axis
           atom     1
          (       -2.01782        0.00000        0.00000 )
          (        0.00000       -2.01782        0.00000 )
          (        0.00000        0.00000       -2.01782 )
           atom     2
          (        2.04775        0.00000        0.00000 )
          (        0.00000        2.04775        0.00000 )
          (        0.00000        0.00000        2.04775 )
================================
My question :
1. why the sum of the effectvie charge is not zero?
2.how to improve the calculation of born effective charge?

The following is my input files:
scf.in
 &CONTROL
                       title = fine,
                      prefix = 'zns'
                 calculation = 'scf' ,
                restart_mode = 'from_scratch' ,
                  wf_collect = .false. ,
                      outdir = './' ,
                  pseudo_dir = './' ,
                     tstress = .true. ,
                     tprnfor = .true. ,
                       nstep = 100 ,
 /
 &SYSTEM
                       ibrav = 2,
                   celldm(1) = 10.204964632,
                         nat = 2,
                        ntyp = 2,
                     ecutwfc = 80,
                       nosym = .false. ,
 /
 &ELECTRONS
            electron_maxstep = 100,
                    conv_thr = 1.D-12,
 /
ATOMIC_SPECIES
    S   32.066   16-S.GGA.fhi.UPF
    Zn  65.409   30-Zn.GGA.fhi.UPF
ATOMIC_POSITIONS crystal
S        0.250000000   0.250000000   0.250000000
Zn       0.000000000   0.000000000   0.000000000
K_POINTS automatic
  16 16 16 0 0 0

ph.in
phonon calculation for ZnS
&inputph
  tr2_ph=1.0d-12,
  prefix='zns',
  epsil=.true.,
  trans=.true.,
  lraman=.true.,
  elop=.true.,
  amass(1)=32.066,
  amass(2)=65.409,
  outdir='./',
  fildyn='ZnS_IR.dynG',
  fildrho='ZnS_IR.drho',
 /
0.0 0.0 0.0

-- 
Y. C. Cheng
Department of Physics
Nanjing University
Nanjing 210093
P. R. China
Tel: 86-25-83592907
Email: yccheng.nju at gmail.com
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