The ABINIT Users Mailing List ( CLOSED )

[Xavier Gonze <gonze@pcpm.ucl.ac.be>]

Dear Fer,

Some technical details ...

(1) TPSS needs the kinetic energy density (space-resolved), that is  
not available in ABINIT at present
(its implementation would not be a lot of work, though)
So, even if the Nanoquanta XC library has it, TPSS cannot be used  
with ABINIT (at present).

(2) More preoccupying, and independent of ABINIT : one cannot compute  
straightforwardly
the Vxc (and Kxc) corresponding to a meta-GGA like TPSS (one needs an  
OEP method - quite unpleasant).
So, I think that in the available implementations (Gaussian 03,  
WIEN), the TPSS is computed
in a one-shot calculation, on top of a LDA (or GGA) density, not self- 
consistently.
So, all optimizations have to be done without the gradients (so,  
without the forces, and stresses).
It is even worse for the computation of the interatomic force  
constants, that one will not be able
to do using the linear-response theory (or with an enormous  
implementation effort)

(3) Hybrid functionals (mixing with Fock contribution - like B3LYP)  
have a much better behaviour if
one does not to do SCF loops as DFT's (meaning again a OEP), but with  
the self-consistent Fock operator.
As the kinetic energy density, the Fock exchange contribution is not  
implemented in ABINIT.
But, unlike the kinetic energy density, the Fock exchange  
contribution is heavy to compute using plane waves...
Still, I have the impression that there is more hope along these  
lines to be able
to compute efficiently accurate (with respect to experiment) coherent  
lattice parameters AND
interatomic force constants.

Not very encouraging in any case ...
Xavier







On 21 Aug 2006, at 22:49, fer@tiziano.phys.washington.edu wrote:

> Hi..
>
>   As I have probably mentioned here before, we have been working on  
> calculating EXAFS Debye-Waller factors using the dynamical matrices  
> generated by abinit. The whole approach works quite well, but we  
> are not totally satisfied by the results we get from either LDA  
> (PW92) or GGA (PBE). For us it is quite crucial getting the  
> interatomic force constant accurately. We have found that they  
> depend a lot on the crystal lattice that we use, and therefore we  
> need to be able to find a way to realiably predict both the crystal  
> lattice and the force constants simultaneously. Its seems that  
> neither PW92 nor PBE can do this.
>
> We got in contact with John Perdew and he suggested we try the meta- 
> GGA TPSS and we have tried to do so with the implementation already  
> available in Gaussian 03. for reasons I don't what to discuss here,  
> this implementation has several drawbacks when it comes to solid  
> states.
>
> Therefore, I was wondering if anyone has implemented TPSS into  
> abinit already. I did a search on the source and found that  
> nanoquantaxc has TPSS, but I really don't know how to integrate it  
> into my binaries nor how to call it. Any experience, instructions,  
> etc, etc, in this regards would be greatly welcome.
>
> Best regards and thanks in advance, Fer.
>
> Ubi dubium ibi libertas.
> ********************************************************************** 
> *********
> Fernando D. Vila                Voice    (206)543-9697
> Department of Physics           Fax      (206)685-0635
> University of Washington        E-mail   fdv@u.washington.edu
> Seattle, WA 98195, USA          WWW      http:// 
> faculty.washington.edu/fdv
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