[Pw_forum] resulting atomic forces when using the "lefield" tag

[S. Sanchez]

Dear Stefano,



One more question related to issue of applying an electric field in a
monoclinic unit cells using the PW.x code. After, I apply a positive
electric field (0.01 a.u.) to my monoclinic system along the x-coordinate
(which is parallel to a-vector); I get the following atomic forces
(Ry/a.u.):



     atom   1 type  1   force =     0.014650         0.000007
-0.000001

     atom   2 type  1   force =     0.014673         -0.000005
-0.000001

     atom   3 type  1   force =     0.015014         -0.000001
-0.000192

     atom   4 type  1   force =     0.015014         -0.000001
0.000192

     atom   5 type  2   force =    -0.014850         -0.000120
-0.001158

     atom   6 type  2   force =    -0.014826         0.000120
0.000167

     atom   7 type  2   force =    -0.014826         0.000120
-0.000163

     atom   8 type  2   force =    -0.014849         -0.000120
0.001156



However, when I apply a negative electric field (-0.01 a.u.), along the same
crystallographic direction, I get the following atomic forces (Ry/a.u.):



     atom   1 type  1   force =    -0.014673         0.000005
0.000002

     atom   2 type  1   force =    -0.014651         -0.000007
-0.000003

     atom   3 type  1   force =    -0.015014         0.000001
0.000187

     atom   4 type  1   force =    -0.015014         0.000001
-0.000191

     atom   5 type  2   force =     0.014824         -0.000119
-0.000169

     atom   6 type  2   force =     0.014852         0.000119
0.001145

     atom   7 type  2   force =     0.014852         0.000119
-0.001149

     atom   8 type  2   force =     0.014824         -0.000120
0.000178



The output of the atomic forces for these two different situations (i.e.,
situation 1 = positive electric field and situation 2 = negative electric
field) is a bit confusing for me, because it seems that the
opposite-sign-atomic-forces-counterparts of atoms 5 and 8 in the positive
electric field situation, are atoms 6 and 7 in the negative electric field
situation. To better understand this, pay close attention to the atomic
forces of atoms 5 and 8 (positive electric field situation) along the
z-direction and compare them with the atomic forces of atoms 6 and 7
(negative electric field situation) along the z-direction. I want to
understand why the components of the atomic forces are swapped if the only
difference in their corresponding input files is the sign of the electric
field. Could please comment on this?



Thanks a lot for your help,

S. Sanchez
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