ABINIT Discussion Forums

I have been trying to calculate the dielectric function of HgS. After checking convergence, I did plot the real and the imaginary part of the dielectric function and the agreement with experiment is very good EXCEPT that at low energies I am getting unusual results. The imaginary part becomes negative, while it should go to zero. At the same time, the real part is going upward when it should got downward. I did check K point convergence as well as Ecut and the result does not change. I think this results is unphysical, basically because if the Img and real part are related through the Kramers-Kroning relation and if the Imag should go to zero, then the Real part should also go to zero. Is there anything I am missing in the calculation? is the a way within Abinit to avoid this problem?

Thanks a lot!

That is very strange.
Which program are you using for the optical spectra?

There are several ways in abinit to get it,
In NC pseudo-potentials, you can use:

optics, conducti or the gw code, there are also external codes as Yambo and DP.
They all in principle should give the same result.

But, you could make a test with another program, to see if the error is related to the program you are using
or to the groud state calculation.

Best
Tonatiuh

Dear Tonatiuh,

let me spell a bit more what is my problem.

I did use the optics code by following the tutorial and changing the variables to fit my problem. In my case, I did have to use
the scissor operator to match the electronic band gap. After the calculation, the agreement with experiment
is really good but as I said, the only thing that I do not like is that the imaginary part of the dielectric
function should be zero up to 1.2 eV and in my case it becomes negative (just a tiny bit) and the real part should decrease
and it increases. In order to check if the problem comes from some type of convergence, I did massage the data and put
at hand zeroes on the imaginary part. By using the Kramers-Kroning relation I was able to obtain the right behavior on the real part.
I will try the Yambo code..

Thanks!