Hello community. I have just started learning how to use The Exciting code. I am interested in second harmonic generation. While doing the related tutorial with GaAs, I attempted to calculate different components of the SHG susceptibility. Given that the group symmetry of GaAs is -43m, all components are zero except for d14. However, after submitting this input file:
<input>
<title>GaAs SHG</title>
<structure speciespath="/home/francis/Software/exciting/tutorials/exciting/species">
<crystal>
<basevect>5.3435 5.3435 0.0000</basevect>
<basevect>5.3435 0.0000 5.3435</basevect>
<basevect>0.0000 5.3435 5.3435</basevect>
</crystal>
<species speciesfile="Ga.xml" rmt="2.0">
<atom coord="0.00 0.00 0.00"/>
</species>
<species speciesfile="As.xml" rmt="2.0">
<atom coord="0.25 0.25 0.25"/>
</species>
</structure>
<groundstate
do="fromscratch"
rgkmax="7.0"
ngridk="11 11 11"
xctype="LDA_PW"
nempty="10"
>
</groundstate>
<properties>
<momentummatrix/>
<shg
wmax="0.3"
wgrid="400"
swidth="0.004"
etol="1.d-4"
scissor="0.0423"
tevout="true"
>
<chicomp>1 1 1</chicomp>
</shg>
</properties>
</input>
I did not obtain zero for d_{11} ; however, the values range from -0.1 to 0.1. In reality, would I never obtain zero due to numerical factors? I conducted several numerical experiments by increasing ngridk. I noticed that by increasing that parameter, \chi^{(2)}_{111} gets reduced; however, it is not yet zero. What am I missing? I am lost. Please, could someone in the community provide insights into this? I would really appreciate it.