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

S. Sanchez lcqsigi at gmail.com
Tue Aug 17 05:15:10 CEST 2010

```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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