[Octopus-users] using MGGA from libxc

[Miguel Marques]

   Dear Vladimir,

   Meta-GGAs are not standard density-functionals. In fact, although by 
virtue of the Hohenberg-Kohn theorem tau is a functional of the density 
n, it is an implicit density functional and not an explicit one (several 
approximations to \tau[n] do exist though). In view of this, there are 
two ways of implementing a meta-GGA in a DFT code:

1) the "correct" DFT way, i.e., treat a meta-GGA as a _orbital_ 
functional and use the machinery of the optimized effective potential 
(OEP) to construct the potential.

2) To approximate the xc potential in DFT by 1/psi \delta E_xc / \delta 
psi^* (from memory, but it should be something like that). This is the 
pragmatic approach used in all quantum-chemistry codes, but it goes away 
from "pure" DFT.

   So, the answer to your question, libxc can not provide \delta tau / 
\delta \rho as this derivative can not be so easily obtained, but that 
derivative is not needed to obtain the potential...

   hope this helped,
   miguel

On 01/22/2011 01:37 PM, Vladimir Nazarov wrote:
> Hello All,
>
> I have a (probably naive)  question about libxc library:
>
> I want to know the xc potential within MGGA.
> For this, I call xc_mgga_vxc. But how to construct v_xc(r)  from the 
> output of
> xc_mgga_vxc: We evidently need  \delta tau / \delta \rho, which is not 
> among the output parameters of    xc_mgga_vxc ?
> (\tau is the kinetic energy density, and \rho is the density).
>
> Thank you very much in advance.
> Vladimir Nazarov.
> _______________________________________________
> Octopus-users mailing list
> Octopus-users at tddft.org
> http://www.tddft.org/mailman/listinfo/octopus-users
>


-- 
Dr. Miguel A. L. Marques
marques at tddft.org
http://www.tddft.org/bmg

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