Yambo Community Forum

Dear Yambo,

https://wiki.yambo-code.eu/wiki/index.p ... tion_(THG)
explains how to choose the external field direction for different X3 (THG).

For my case, I do not know how to setup E-filed directions for X*xyy, X*xyx, and X*yyx.
I am wondering for a given X component (X*abc), how to decide E-filed direction, i.e.,
coefficients of the linear combination of the Cartesian axes. For example, is E-field direction
of (110) for X*xyy, X*xxy, X*xyx, or another component?

Any suggestions and/or references will be great.

Thanks,
Jie

Dear Jie

this is a very good question. I'm not sure I have a good answer, but you can try

1) if can put the field in the direction (1, 1, 0 ) and then study the Xhi3 along x and y
I guess it should contain:

X3_x = X*xyy + X*xxy + X*xxx + X*yyy + X*yyx + X*yxx + X*xyx + X*yxy
Y3_x = Y*xyy + Y*xxy + Y*xxx + Y*yyy + Y*yyx + Y*yxx + Y*xyx + Y*yxy

some of these will be zero or equivalent by symmetries, and maybe you can extract then one you are searching for.
And/our you can sum/subtract the different X3 you get from Yambo using fields in different directions, for example for a field in x you get

X3_x = X*xxx
X3_y = Y*xxx
and in y you get

X3_x = X*yyy
X3_y = Y*yyy

these can be subtracted from the previous ones to remove components you do not want.

2) you can put two different external fields, and then try to post-process the result in some smart way
see the example of pump and probe
https://wiki.yambo-code.eu/wiki/index.p ... _and_Probe
in this case you can also think to use Richarson extrapolation, changing the intenty of the fields in x or y direction to eliminate
some components of X3. See the example for two-photon absorption
https://wiki.yambo-code.eu/wiki/index.p ... absorption

best
Claudio

ps I think the first solution is the best, you fied calculate X3_xxxx and X3_yyy then the X3 with the field in (1,1,0)
and remove this diagonal components, the rest should be simplified with symmetries

Dear Claudio,

Great. Your answers are very helpful.

Thanks,
Jie