The ABINIT Users Mailing List ( CLOSED )

["Ramirez-Cuesta, AJ \(Timmy\)" <A.J.Ramirez-Cuesta@rl.ac.uk>]

Guys,
I found that most of the times LDA works OK if the free energy is
minimised instead of just the energy. This minimisation of the free
energy is done calculating the full vibrational density of states and
using quasiharmonics lattice dynamics. It is easily done with abinit and
anaddb!

I focused on INS (inelastic neutron scattering) spectra, since I work
with one of such machines. And all systems I considered included
hydrogen!
When using LDA, the series of simple alkali hydrides, Li-Cs H, actually
give good agreement in geometry and vibrational properties. Same applies
to polyethylene!

The argument for me goes this way, if you get the right lattice
parameters minimising the energy that is essentially wrong since you
have ignored thermodynamics (zero point energy contributions and thermal
contributions) We always try to use quasiharmonic lattice dynamics.
Although to be fair that is usually impossibly expensive
computationally! 

But I think that there is a serious case for a better assessment and
thorough investigation of this fact.

See
1- LDA or GGA? A combined experimental inelastic neutron scattering
and ab initio lattice dynamics study of alkali metal hydrides, Barrera,
G. D.; Colognesi, D.; Mitchell, P. C. H.; Ramirez-Cuesta, A. J. Chemical
Physics 2005, 317, 119-129.

2.- Mitchell, P. C. H.; Parker, S. F.; Ramirez-Cuesta, A. J.; Tomkinson,
J. Vibrational Spectroscopy with Neutrons: with applications in
Chemistry, Materials Science and Catalysis; World Scientific: London,
2005

3.- Hydrogen dynamics in heavy alkali metal hydrides
obtained through inelastic neutron scattering, Auffermann, G.; Barrera,
G. D.; Colognesi, D.; Corradi, G.; Ramirez-Cuesta, A. J.; Zoppi, M.
Journal of Physics-Condensed Matter 2004, 16 , 5731-5743.

4.- The Ultimate Modulus of Polyethylene. Gustavo D. Barrera
Stewart F. Parker and Anibal. J. Ramirez-Cuesta, Philip C. H. Mitchell.
Accepted for publication in Macromolecules (2006).

Regards


-------------------------------------------
Dr. Anibal Javier Ramirez-Cuesta
Scientist
TOSCA
ISIS Facility
Rutherford Appleton Laboratory
Building R3, Room 1-42
Chilton
Didcot
Oxon
OX11 0QX
UNITED KINGDOM
email: a.j.ramirez-cuesta@rl.ac.uk
Office: +44 (0)1235-446510
Cabin: +44 (0)1235-446356
Mobile: +44 (0)778 710 5335
Fax: +44 (0)1235-44-5720
--------------------------------------------
 

-----Original Message-----
From: Masayoshi Mikami [mailto:mmikami@rc.m-kagaku.co.jp] 
Sent: 22 February 2006 03:22
To: forum@abinit.org
Cc: yanqingb@mails.gucas.ac.cn
Subject: [abinit-forum] references about vdW benchmarks

Dear Qing-Bo Yan,

Please let me relay your question to the ABINIT forum ML.

Yes, It is a sort of "rule of thumb".
Embrassingly, LDA works well with C60/graphite, but GGA will not.
But I cannot remember papers that point out the reason
why GGA fails for such systems (and LDA works ...).
I have believed that the successful case with LDA should be regarded as 
"just fortunate" ...

Subscribers, which paper should be recommended for this ?
It might be a good occasion to refresh our memories on the status-quo...

Regards,
Masayoshi

P.S. Most papers in journals will focus on their successful stories,
not their failures ... ;-)

On 2006/02/22, at 11:57, yanqingb@mails.gucas.ac.cn wrote:

> Dear Masayoshi Mikami,
>
> I am a ABINIT leaner, and I am very interesting in one of your replies

> on the ABINIT mailing list. It reads as follow:
>
> .. By chance, LDA may reproduce sort of van der Waals, for example, in

> graphite/fullerene systems (i.e. lattice constant may be anyhow 
> reproduced with rather high energy cut-off), but it seems very 
> "empirical"... Who knows the reason ? In fact, I also noticed that GGA

> cannot not reproduce the van der Waals in graphite and C60 fcc solid. 
> ...
>
> Would you please give me some relative references for further 
> investigating? Thank you very much!
>
> Your sincerly
>
> Qing-Bo Yan
> -----------------------------------------------
> Qing-Bo Yan
> Ph. D. Candidate
> College of Physical Sciences,
> Graduate University of the Chinese Academy of Sciences
> Beijing, China
> -----------------------------------------------
>