[Pw_forum] different born effective charges

[VineetKumar Pandey]

in the output file of ph.x executable, there is given effective charges
calculated by different methods (1. through force per unit electric field,
2. through polarisation per unit cell ). and both are different. which one
is reasonably good?

 Effective charges (d Force / dE) in cartesian axis

           atom      1   Cd
      Ex  (        3.96115        0.02292       -0.01695 )
      Ey  (        0.32444        3.81353        0.00947 )
      Ez  (        0.04898        0.00111        0.58030 )
           atom      2   I
      Ex  (       -1.95631        0.00202       -0.00053 )
      Ey  (        0.24237       -2.15621       -0.01983 )
      Ez  (        0.04027       -0.00240       -0.35984 )
           atom      3   I
      Ex  (       -2.00331       -0.02499        0.01617 )
      Ey  (        0.21596       -2.10782        0.00910 )
      Ez  (        0.04265        0.00146       -0.35983 )

          Effective charges (d P / du) in cartesian axis

           atom      1   Cd
      Px  (        3.96020        0.02291       -0.01604 )
      Py  (        0.02291        3.98632        0.00926 )
      Pz  (       -0.00184        0.00108        0.60842 )
           atom      2   I
      Px  (       -1.95983        0.00203       -0.00031 )
      Py  (        0.00216       -2.01825       -0.01912 )
      Pz  (       -0.00011       -0.00226       -0.30355 )
           atom      3   I
      Px  (       -2.00682       -0.02500        0.01640 )
      Py  (       -0.02510       -1.97112        0.00982 )
      Pz  (        0.00192        0.00127       -0.30355 )
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