[Pw_forum] modified explicit exchange potential

[degironc]

Helen wrote:
> Thank you Paolo and Lorenzo to the replies to my previous e-mail. I 
> think I didn't explain what I wanted to modify clearly enough. We 
> modify the explicit exchange to include a descreened potential:
> K_x[n]=-0.25 Int[ P[n](r,r')^2 * u(r-r') ] d3r d3r'
> where P[n](r,r') is the density matrix of non-interacting particles 
> and u(r-r') is the descreened interaction (normally the coulomb 
> interaction u(r)=1/r, but in our case a different function).
>  
> Which files do I need to change for the modified K_x and its 
> potential? Is it exx.f90 in PW? These explicit exchanges are also used 
> in Hartree-Fock and B3LYP calculations.
>  
> Thank you for your help,
> Helen Eisenberg
Yes, the part of the code performing HF-like exchange is in exx.f90.
You need to compile the code specifying -DEXX at compilation time... HF, 
B3LYP and PBE0 are implemented (but no corresponding pseudopotential are 
available so one need to use some PBE or LDA one and overwite the XC by 
the input_dft flag in the system namelist) ...
Beware: it is extreeeemely slow.

As you wll see the EXX part requires some work to be made more useful... 
if you are interested in working in that line let me know... :-) .. by 
the way, full affiliation would be appreciated.

Stefano de Gironcoli
SISSA and DEMOCRITOS, Trieste, Italy