[MMTK] implementing new force field

[Olivier Delaire]

Dear MMTK users,

I am trying to extend the force-fields implemented in MMTK to treat the 
case of a crystal vibrating around its equilibrium configuration. The 
type of force-field I want to implement would be an expansion of the 
energy in terms of small displacements of nuclei from their equilibrium 
sites (Taylor-type expansion).
If I expand the energy up to 2nd order in the nuclear coordinates, then 
I should get quadratic on-site terms corresponding to 'Einstein 
ocsillators', and already implemented as one of the harmonicFF examples 
in the distribution. Besides the on-site terms, one should also get the 
cross-terms u_i x u_j , where u_i is the displacement amplitude for atom 
i (here in 1D).
Now, I would want to implement such a force-field up to higher orders 
(say 3 or 4), and of course I would have to restrict the range of each 
type of interaction in order not have so many free parameters.

I have been looking in to the Python and C force-field classes, but 
these are rather intimidating. Presently, it seems to me that the 
nonbonded.c file could be extended to treat that kind of force-field, 
but I am unsure wether assumptions in other places in the package will 
break if I try to modify it... Also, I am not entirely sure wether I 
should start with the 'bonded' or 'nonbonded' types of force fields.

Does anyone have any experience in implementing new force-fields for 
MMTK? Where should I start?
Your help is greatly appreciated.

Thanks,
Olivier

-- 
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Olivier Delaire, Ph.D.

California Institute of Technology
Division of Engineering and Applied Science
Materials Science MC 138-78
1200 E. California Boulevard, Pasadena, CA 91125, USA

Phone : (626) 395 3627
Fax : (626) 795 6132
Email : delaire at caltech.edu
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