CCL: Forces between atoms in molecule

 Sent to CCL by: Mariusz Radon [mariusz.radon[]gmail.com]
 On 30.01.2013 05:19, alireza zeinalinezhad ar.zeinali ~~ yahoo.com wrote:
 > Sent to CCL by: "alireza  zeinalinezhad" [ar.zeinali||yahoo.com]
 > Dear ccl-netters,
 > Does anyone know how to find/print the forces between atoms in
 molecule/cluster using g09.
 > thank you
 > Alireza
 Hi Alireza:
 In order to obtain forces acting on the atoms you can use a small trick:
 just do a single step of geometry optimization, i.e., Opt(MaxCycles=1).
 The forces would be zero, of course, in the equilibrium geometry.
 However, to calculate forces acting on atoms is not really the same as
 to calculate "forces between atoms in a molecule/cluster." If I
 understood you correctly, I am afraid that what you want to obtain
 cannot be uniquely defined because the total energy in quantum chemistry
 is _not_ a pairwise inter-atomic potential. Thus, I believe it is not
 straightforward to decompose the resulting forces into a sum of
 contributions, one from each pair of atoms.
 Regards,
 Mariusz Radon>
 >
 --
 Dr Mariusz Radon, Ph.D.
 Coordination Chemistry Group
 Faculty of Chemistry
 Jagiellonian University
 ul. Ingardena 3, 30-060 Krakow, Poland
 http://www.chemia.uj.edu.pl/~mradon