Hi Tobias,
(1) Unfortunately for 7 different atoms you need explicitly specify the
28 unique pairwise potentials. This is tedious, which is why I have a
program that does it for me. One thing to note in the release versions
of SIESTA there is only the memory space for 10 pair potentials. If you
want to correctly use Grimme's dispersion correction you will have to
change maxMMpot in the molecularmechanics.F90 and re-compile the
program. If you don't do this SIESTA will only use the first 10
potentials that you specify.
(2) Grimme uses J nm^(6) mol^(-1), SIESTA uses eV angstrom^(6), so the
conversion factor is approx. 10.3641
(3) I am not sure about the 1.66. From Grimme's 2006 paper for PBE
this should be 0.75. The D value is the same as in Grimme's paper.
I hope this helps you.
Brad Wells.
Monash University.
On 5/02/2014 1:07 AM, Kraemer, Tobias wrote:
Dear all,
I would like to try out adding Grimme's dispersion correction to my
calculation. From the manual and the list archive
I understand that this can be done, however, some points are unclear
to me.
The parameters need to be defined in the MM.Potentials block, and if I
am not mistaken, this needs to be done
for each pair wise interaction between atom types. For my system
containing 7 different atoms, this seems to be
quite tedious. Is there a more straight forward way of doing this?
Certainly I could restrict the number to only include
nearest neighbours, but this is difficult to decide a priory. So,
there would be a large number of pairs that would need to be
considered here.
Nonetheless, if I should do it this way how would I translate between
different units. It appears that the original Grimme paper reports
these parameters in Jnm^(6)mol^(-1) which differs from SIESTA standard
units (I stumbled upon this discussion in the archive, however, I
couldn't find
A conclusive answer. Unfortunately I also seem to be unable to recover
the post from the archive, despite some time spent searching).
My last question addresses the 2 parameters MM.Grimme.D and
MM.Grimme.S6. The latter is by default set to 1.66 for a DZP basis,
however,
For which functional is this the default. Can this value be ised
straight away with PBE? Where does this value come from?
Likewise, can the default value for .D be used straight away?
Thanks for your help....
Tobias
________________________________
Dr. Tobias Kraemer
Research Associate
Institute of Chemical Sciences
School of Engineering & Physical Sciences
Heriot-Watt University
Edinburgh EH14 4AS
United Kingdom
* [email protected]
( +44 (0)131 451 3259
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