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

[Pavel Ondračka]

Dear Atefe Marasi,

to add on top of what Xavier said, and thus similarly to him I'm also
assuming that by "infinite epsilon, i.e., dielectric constant at high
frequency" you mean the electronic part the dielectric tensor. Or in
other words the part from the electronic excitations. That you can get
with the optic, joint and kram commands.

Regarding the band gap, if you have insulator, the mBJ potential should
be a reasonable starting choice. The band gap values are OK and even
though the band dispersion and thus the momentum matrix elements are
not so good, it does not matter that much as the standard optic
calculation neglects excitons anyway. However even if you don't get the
proper shape of the imaginary dielectric function, in my experience
this does not matter that much if you are interested in the real part
of the dielectric function at E=0 (or at least reasonably far below the
band gap). I used mBJ+optic quite a lot for refractive index
calculations in vidible range for materials with band gap as low as 3eV
and even there it worked quite OK.

What is very important is to increase the maximum energy emax value in
lapw1, optic and joint to be sure you include enough states to properly
account for the high energy processes. Otherwise you will get some
underestimation of the real part of the dielectric function at low
energies.

Best regards
Pavel

On Mon, 2021-11-29 at 11:46 +0100, xavier rocquefelte wrote:
> Dear Atefe Marasi,
> Infinite epsilon means that you extrapolate the epsilon value for the
> zero of energy. 
> You must plot the real part of the dielectric function to properly
> estimate this value. 
> You must be careful because if you have a band gap and a bad
> description of the gap value ... the estimation will be not good at
> all. 
> Regards
> Xavier
> 
> On 29/11/2021 11:38, Atefe Marasi wrote:
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