Yambo Community Forum

Dear Davide,

Notice that the peak is negative if the absorption is constructed from the "retarded" four point propagator.

This is exactly what I observed by going from "resonant" to "retarded" as mentioned before.

The dielectric function contains both absorption and stimulated emission (we usually call it "absorption" just because the emission channel is always zero from the ground state). Then, if light is emitted, the dielectric function must be negative at that energy.

This is very interesting, since I thought that for stimulated emission one needs to calculate contributions from an equation specifically derived for stimulated emission which is not coded in YAMBO.
According to what you say, this makes the negative energies in the positive range fully physical but only if we assume that we have the wrong GS, and consider it as an excited state.
In VASP, I don't see any negative energies in the positive range when including anti-resonant contributions. It is possible that VASP gives incorrect results regarding this issue.

Since one cannot update the wave functions in YAMBO, I will check it with hybrid-DFT as my starting point and look what happens, whether I still get the negative energies or not.
I will let you know.

Best,
Martin

Since one cannot update the wave functions in YAMBO, I will check it with hybrid-DFT as my starting point and look what happens, whether I still get the negative energies or not.
I will let you know.

I think you need specific equation if you want to isolate the stimulated emission contribution from the absorption (beyond the Tamm-Dancoff approximation it is non trivial).
The sum of the two is embodied in the response function.
Also if you want to compute the spontaneous emission you need specific equations.
Pedro Melo and Andrea Marini are working on that, you can find some publications in the literature.

Since one cannot update the wave functions in YAMBO, I will check it with hybrid-DFT as my starting point and look what happens, whether I still get the negative energies or not.
I will let you know.

Remember that even just updating the energies from KS to QP you are "changing the GS".

D.

Dear Davide,

Thanks.

Remember that even just updating the energies from KS to QP you are "changing the GS".

I agree with you, however you could still get negative spurious energies due to wrong GS even if you update the energies.
Therefore for me personally a real change of the GS is connected to a change of the structure of the wave functions.
Since this system contains transition metals, an update of the wave functions is probably a necessity.

Best
Martin

Dear Martin,

Since this system contains transition metals, an update of the wave functions is probably a necessity.

Yes, this can be an issue when dealing with transition metals, we are actually working to allow Yambo to start from a DFT+U calculations,
we still need some coding and I hope to have it ready soon.

Best,

Daniele

Dear Daniele,

Thanks for your reply.
I am personally not a big fan of DFT+U, due to somehow arbitrariness of U.

Is update of the wave functions in the development version of YAMBO not possible?
It seems to be quite straightforward in VASP and Abinit.
I am wondering why YAMBO still lacks this important feature.
Wave function update can also have a tremendous impact on optical properties.

Best wishes,
Martin

Dear Martin,
I understand your skepticism about DFT+U, but in many case it is the cheaper solution.
Yambo has the possibility to update the wave functions by performing self consistency in the COHSEX approximations but I'm afraid
this part it is not yet in the GPL release. Hopefully it will be released soon, I agree with you that in many case it is an essential ingredient.

Best,

Daniele