[Pw_forum] a possible problem in the routine add_efield.f90

[hanghui chen]

Dear PWSCF users,
              In my previous email, I reported that if I calculated a
symmetric
   system (both the material and the external sawtooth potential are
   symmetric), I found that the forces are not symmetric. I think I may find
   the reason.
        The total force in the presence of sawtooth potential has two parts:
   one is from the variation of Kohn-Sham functional; the other is from
   sawtooth potential which in the code is "forcefield" (see the routine
   forces.f90). The additional force is calculated by the routine
   add_efield.f90. However in routine add_efield.f90, it only calculates the
   magnitude of the force but does not specify the direction (the additional
   force only has one direction).  Therefore all the forces in the 'reversed
   region' are not correctly calculated because the direction of
'forcefield'
   is NOT reversed as is the external electric field.
        I sincerely hope that the people who wrote the routine
add_efield.f90
   can reexamine the code and add the directions to the "forcefield".
        No offense intended.
        Thank you very much.
   Hanghui Chen
   Department of Physics, Yale University
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