no bound exciton

Deals with issues related to computation of optical spectra, in RPA (-o c) or by solving the Bethe-Salpeter equation (-o b). Includes local field effects, excitons, etc.

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xixi
Posts: 30
Joined: Tue Mar 24, 2009 1:08 pm

no bound exciton

Post by xixi » Sun Apr 05, 2009 10:25 am

Dear all;
I always met such cases, the GW gap is smaller than the BS energy, that means the binding energy is minus, in some case, I strength the K sampling and the problem solved, but in many other cases, it didn't work, well, could anyone give some advice on this issue? how to determine a system whether can bind exciton or not? Thanks

Xi Zhu
Nanyang University
Singapore,639798

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andrea marini
Posts: 447
Joined: Mon Mar 16, 2009 4:27 pm
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Re: no bound exciton

Post by andrea marini » Sun Apr 05, 2009 1:08 pm

xixi wrote:Dear all;
I always met such cases, the GW gap is smaller than the BS energy, that means the binding energy is minus, in some case, I strength the K sampling and the problem solved, but in many other cases, it didn't work, well, could anyone give some advice on this issue? how to determine a system whether can bind exciton or not? Thanks


The fact that the GW gap is smaller that the lowest BS energy is not a problem, in general. There is no strict, exact rule to determine whether your system is gonna have a bound exciton. So, please, give us more details about your system: is it a metal, a semicondutor or an insulator. How large is the DFT/GW gap ? Home many atoms, which type of atoms .... and so on.

From a very very general point of view you could say

  1. larger the gap, larger the probability to have a bound exciton
  2. larger the dis-homogeneity, larger the probability to have a bound exciton
  3. metal = no bound excitons
  4. ...

Cheers

Andrea
Andrea MARINI
Istituto di Struttura della Materia, CNR, (Italy)

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