[Wien] Static Dielectric constant of CsI..

[Fred Nastos]

On March 18, 2004 08:29 am, Nitya Nath wrote:
> Dear Wien Users,
>
> We calculated the static dielectric constant of CsI after executing 'kram'
> to Img dielectric function. The value found was 3.80 insteed of the quoted
> value 6.59 in Ashcroft's book page no 553. We used 100000 total k-points
> in BZ and also energy grid is 0.0136 eV for joint calculation. What could
> be the possibility for this difference?

I'll list some of them.

First the ones can easily overcome:
- You don't have the correct "scissor-shift" value.  The scissor correction
ends up regidly-shifting the imaginary spectrum higher up in frequency.
So, from the K-K relation you see that the real part of the dielectric 
constant will depend on the value of the band gap.  Find the correct
band-gap and shift your spectrum.

Now the ones you cannot easily overcome:
- phonon contribution.  CsI is ionic.  As far as I know, you are calculating
only the electronic contribution.  Note that the entire section you refered
to in A&M is about phonon contributions to static dielectric constant.
- local field effects.  Loal field effects are excluded in the calculation.
In some materials they are a small correction, but in others they can
be much larger (20-30%?)