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[fer@tiziano.phys.washington.edu]

(NOTE: I tried sending this message by replying to the sender but SYMPA 
rejected my message, any idea why??)

Hi, and thanks for your response..

On Tue, 22 Aug 2006, Andrew M. Rappe wrote:

> Dear Fernando,
> 
> What category of material or system are you studying?

John Rehr (my boss) is goading me to study every system under he sun. I'd be
happy to get Cu right. :-) Right now, to get a handle of the trends, I have
been calculating noble metals (Cu, Ag, Au, Pt, etc), I also tried Fe, but I
have some problems there getting conjugate gradient to converge for some of
the q distortions. I have also calculated C(Dia), Si and Ge. In these cases
I actually get quite good results, so I've been concentrating in getting the
metals right.

> LDA is renowned for giving pretty accurate interatomic force constants
> even though it is not an exceedingly accurate functional.  There are
> many things that must be correct for you to get accurate interatomic
> force constants.  It doesn't strike me as likely that the functional
> is your dominant error (although I suppose it's possible).

This is our experience also. We have come to the conclusion that LDA does a
pretty decent job regarding the ifc's. The caveat is getting the lattice
constant right.

Let me give you an example: for Cu, a change of about 1% in the lattice
constant generates a change of about 5% in the centroid of the VDOS. Such a
change in frequency implies a change of about 20% in the force constants,
and since the DW factors are directly related to the ifc, we get an error of
%20 in them.

> Can you be specific about what systems you are studying, and what your
> computational approach is?  Perhaps we can suggest other sources of
> error.

I hope the information above helps, and thanks for the interest.

Fer.

> Sincerely,
> Andrew M. Rappe
> 
> On Tue, Aug 22, 2006 at 08:39:02AM +0200 or thereabouts, Anglade
> Pierre-Matthieu wrote:
> > Hi,
> > 
> > Have you tryed a look at the different functionnals readily available
> > in abinit ? For instance I have heard that using "ixc=9" may give some
> > much more accurate results.
> > 
> > regards
> > 
> > PMA
> > 
> > On 8/21/06, fer@tiziano.phys.washington.edu
> > <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
> > > **********************************************************************
> > > *********
> > > 
> >