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<p class="MsoNormal">Dear Stefano,</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">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.):</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal"><span style=""> </span>atom<span style=""> </span>1 type<span style="">
</span>1<span style=""> </span>force =<span style=""> </span>0.014650<span style=""> </span>
0.000007<span style=""> </span>-0.000001</p>
<p class="MsoNormal"><span style=""> </span>atom<span style=""> </span>2 type<span style="">
</span>1<span style=""> </span>force =<span style=""> </span>0.014673<span style=""> </span>-0.000005<span style=""> </span>-0.000001</p>
<p class="MsoNormal"><span style=""> </span>atom<span style=""> </span>3 type<span style="">
</span>1<span style=""> </span>force =<span style=""> </span>0.015014<span style=""> </span>-0.000001<span style=""> </span>-0.000192</p>
<p class="MsoNormal"><span style=""> </span>atom<span style=""> </span>4 type<span style="">
</span>1<span style=""> </span>force =<span style=""> </span>0.015014<span style=""> </span>-0.000001<span style=""> </span> 0.000192</p>
<p class="MsoNormal"><span style=""> </span>atom<span style=""> </span>5 type<span style="">
</span>2<span style=""> </span>force =<span style=""> </span>-0.014850<span style=""> </span>-0.000120<span style=""> </span>-0.001158</p>
<p class="MsoNormal"><span style=""> </span>atom<span style=""> </span>6 type<span style="">
</span>2<span style=""> </span>force =<span style=""> </span>-0.014826<span style=""> </span>
0.000120<span style=""> </span> 0.000167</p>
<p class="MsoNormal"><span style=""> </span>atom<span style=""> </span>7 type<span style="">
</span>2<span style=""> </span>force =<span style=""> </span>-0.014826<span style=""> </span>
0.000120<span style=""> </span>-0.000163</p>
<p class="MsoNormal"><span style=""> </span>atom<span style=""> </span>8 type<span style="">
</span>2<span style=""> </span>force =<span style=""> </span>-0.014849<span style=""> </span>-0.000120<span style=""> </span> 0.001156</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">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.):</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal"><span style=""> </span>atom<span style=""> </span>1 type<span style="">
</span>1<span style=""> </span>force =<span style=""> </span>-0.014673<span style=""> </span>
0.000005<span style=""> </span> 0.000002</p>
<p class="MsoNormal"><span style=""> </span>atom<span style=""> </span>2 type<span style="">
</span>1<span style=""> </span>force =<span style=""> </span>-0.014651<span style=""> </span>-0.000007<span style=""> </span>-0.000003</p>
<p class="MsoNormal"><span style=""> </span>atom<span style=""> </span>3 type<span style="">
</span>1<span style=""> </span>force =<span style=""> </span>-0.015014<span style=""> </span>
0.000001<span style=""> </span> 0.000187</p>
<p class="MsoNormal"><span style=""> </span>atom<span style=""> </span>4 type<span style="">
</span>1<span style=""> </span>force =<span style=""> </span>-0.015014<span style=""> </span>
0.000001<span style=""> </span>-0.000191</p>
<p class="MsoNormal"><span style=""> </span>atom<span style=""> </span>5 type<span style="">
</span>2<span style=""> </span>force =<span style=""> </span>0.014824<span style=""> </span>-0.000119<span style=""> </span>-0.000169</p>
<p class="MsoNormal"><span style=""> </span>atom<span style=""> </span>6 type<span style="">
</span>2<span style=""> </span>force =<span style=""> </span>0.014852<span style=""> </span>
0.000119<span style=""> </span> 0.001145</p>
<p class="MsoNormal"><span style=""> </span>atom<span style=""> </span>7 type<span style="">
</span>2<span style=""> </span>force =<span style=""> </span>0.014852<span style=""> </span>
0.000119<span style=""> </span>-0.001149</p>
<p class="MsoNormal"><span style=""> </span>atom<span style=""> </span>8 type<span style="">
</span>2<span style=""> </span>force =<span style=""> </span>0.014824<span style=""> </span>-0.000120<span style=""> </span> 0.000178</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">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?</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Thanks a lot for your help,</p>
<p class="MsoNormal">S. Sanchez</p>
<br><br>