Some explainations on the variable "GrFnTpXd"

[iacomino]

Dear Yambo Developers,
we would like to have some explainations on the meaning of
the various options for the variable GrFnTpXd.
In particular the difference between "r" and "c".
We think that the correct option for a spectrum calculation
is "c", can you confirm it?
If this is the case, we would like to understand the reason
why the default option is "t" with a "yambo -o c" type of calculation.
thank you!

Sincerely,
Pierluigi Cudazzo and Amilcare Iacomino

UPV San Sebastian ES
Comune di calle easo 25, San Sebastian ES

[andrea marini]

Hey guys, the difference is easy; r stands for resonant, a for antiresonant, t for time-ordered and c for causal. Yambo is mainly a many-body code, this is way the time-ordered Green's functions are used by default. TDDFT, instead, should be formally done with causal Green's functions. However, you will not see any difference between using causal and T-ordered ordering. It is only a matter of conventions.

Instead, if you use the resonant/antiresonant options is like using the Tamm-Dancoff approx for positive/negative frequencies....

Hope I was clear enough.

Andrea