[Wien] How to find the exact value of infinte epsilon?

[Fecher, Gerhard]

What is the wavelength at zero energy ?

Ciao
Gerhard

DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
"I think the problem, to be quite honest with you,
is that you have never actually known what the question is."

====================================
Dr. Gerhard H. Fecher
Institut of Physics
Johannes Gutenberg - University
55099 Mainz
________________________________________
Von: Wien [wien-bounces at zeus.theochem.tuwien.ac.at] im Auftrag von Atefe Marasi [13marasi at gmail.com]
Gesendet: Montag, 29. November 2021 23:25
An: wien at zeus.theochem.tuwien.ac.at
Betreff: Re: [Wien] How to find the exact value of infinte epsilon?

Dear Xavier

Thank you for your prompt reply.

> Infinite epsilon means that you extrapolate ...

Do you mean that infinite epsilon (epsilon_oo) is nothing more than epsilon zero (epsilon_0)?

> You must plot the real part...
I have plotted the real part of epsilon in z direction(for BaTiO3), as would be seen from the following link:
https://imgurl.ir/uploads/p793339_infinite_epsilon.jpg
As expected, the above figure shows that the real part of the epsilon tends to unity at high frequency. From this plot, the epsilon_0 is about 6.5, but I am still not sure that whether infinite epsilon is really equal to epsilon_zero (epsilon_oo =? epsilon_0).

> You must be careful because if you have a band gap and a bad description of the gap value ...

Thank you for this hint. I will try to obtain a suitable band gap by TB-mBJ functional. However, at this step, I need to find out the difference between epsilon_oo and epsilon_0. In my opinion, which I am not sure about it, epsilon_00 would be evaluated at infinite energy, while epsilon_0 would be evaluated at zero energy, as deduced  from their names.
Thank you for your nice cooperation.

Best Regards
Atefe Marasi