[CPMD-list] Qusestion about the functionality of &VDW

[Robert Williams]

Dear List,

With respect to VDW CORRECTION,
it is not clear from the manual or
from the examples how
the parameters should be chosen.

Of the parameters:
type, i, j, \alpha_{ij}, R^{ij}_m, \beta_{ij}
only the "type" of C6 is explained.
(My notation is in TeX.)

i,j: I gather from the source that i and j are
the indexes of the atom types in the order that
they are listed in the &ATOMS section.

\alpha_{ij}: has to be
calculated by hand using tables I and II and equation 7
from Elstner et al., I gather.
However, when I do this I do not get values
that are close to the values used in the
cpmd/contrib/CPMD-test/vdw examples.  Please someone
correct me if I've made a dumb mistake.
It would be very
useful to know what units are expected, since
three of the papers I've read all use different
units for the C_6 coefficients, J nm^6 mol^{-1} (S. Grimme),
\AA^6 kcal/mol (S. Serra et al. Chem. Phys. Let. 331 (2000) 339-345,
one of the references Axel refers to below),
and in the Elstner paper, eV/\AA^6 (!?
This must surely be a typographical
error in this paper.  It should be eV \AA^6/mol.)
The source is not informative here.


The reference, M. Elstner et al.,
gives a damping term in equation 8
that appears to be different from that given
in the cpmd manual under "VDW CORRECTION",
so it is not clear how R^{ij}_m is determined.
Likewise, \beta_{ij} (Elstner's "d*" in his eqn. 8),
is also in question because the form of the damping
terms is quite different in the manual compared to
Elstner et al., a source of concern.


After a cursory reading
of the cpmd source, and running the examples,
I am a little confused
about how to choose atom types (i,j), since the types
chosen for the ch4-ch4 calculation in the
example both appear to be for carbon,
where i and j are 1.  Should not the
dispersion terms here be for H?


I'm waiting for an interlibrary loan to read
the references by Le Sar (1984, 1987),
and Elstner does refer to several other
earlier papers that I've not seen yet, so
perhaps I'm a little premature to submit
this question to the list.   Still,  the
manual should probably contain a little
more guidance about the parameters,
and about the difference between the implemented
damping term and the one given in the reference.

Can anyone clarify this issue?

The most comprehensive recent VDW paper I've
read is: S. Grimme, J Comput Chem 25: 1463-1473, 2004.

Best regards,
Bob Williams

Axel Kohlmeyer wrote:
> On Tue, 7 Dec 2004, Song Huajie wrote:
> 
> HS> Dear CPMD experters,
> 
> dear hj song,
> 
> please have a look at the 'vdw' subdirectory of the
> CPMD-test archive. it contains two examples for the 
> use of this feature.
> 
> you may also want to have a look at 
> 
> chem. phys. lett. 331(2000), 339-345
> and
> chem. phys. lett. 360(2002), 487-493
> 
> for two successful applications of the VDW correction.
> 
> HS> To study the dispersion bound systems (organic clusters and solids), I
> HS> intend to use the functionality (&VDW) of CPMD that adds van der Waals
> HS> (dispersion) term to DFT. Unfortunately, The Manual of CPMD does not
> HS> offer more information about it. I would like to know whether or not
> HS> to activate the &VDW when performing energy minimization and (or)
> HS> molecular dynamics. I am very hopeful that he who has been used this
> HS> &VDW can response or offer some input files for carring out this
> 
> i have not used the feature myself, but i only makes sense to me, 
> to use it consistently for all calculations.
> 
> best regards,
> 	axel kohlmeyer.
> 
> HS> functionality. Your help will be appreciated.
> 
> HS> With best regards,
> HS>      H.-J. Song 
> HS> 
> HS> 
> HS> 
> HS> 
> HS> _______________________________________________
> HS> CPMD-list mailing list
> HS> CPMD-list at cpmd.org
> HS> http://cpmd.org/mailman/listinfo/cpmd-list
> HS> 
> HS> 
> 

-- 
Dr. Robert Williams
Dept. of Biomedical Informatics
Uniformed Services University
301-295-3568