[CPMD-list] Get intermolecular interaction by cpmd

[Axel Kohlmeyer]

On Fri, 19 Oct 2007, Younes Ansari wrote:

YA> Dear Dr. Kohlmeyer Thanks for your quick reply.Actually you are 
YA> right. I want to know how does electronic effects affect the 
YA> classical forcec field.In particular, If we add a ?CH3 group to a 

what you are asking touches the very principles of how
force fields are built: in traditional force fields functional 
units (=residues) are essentially independent, i.e. follow the 
principles of "chemical intuition". current developments try
to include polarization effects, but that is quite difficult.

YA> compound, how it is possible to parametrized an accurate force field 
YA> by an ab initio method? Best regards,

please _read_ the literature about how force fields like AMBER,
CHARMM, GROMOS, OPLS and so on are parameterized and you can
see how they include ab initio calculations to derive the 
charge distribution of the respective compound/fragments.

the term 'accurate force field' is a bit of a contradiction in
itself. since you integrate out the electronic structure to
represent it by point charges, you trade accuracy for speed.
on the other hand, due to the empirical nature you can include
effects that would otherwise be missing, e.g. dispersion
interactions that are massively underrepresented in normal 
DFT calculations.

cheers,
   axel.



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-- 
=======================================================================
Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
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