Subject: Bond strenghts

From: <[email-masked]>
Date: Mon, 23 May 1994 11:12:51 -0500 (EST)

Dear co-neters:
P. S. Subranian asks about how to calculate "bond strengths"  There are not
many theories that give a rigorous way to calculate bond strengths in an
unambiguous way.  Dissociation energies are, in my opinion, a very different
thing.  There is however a rigorous formalism that yields "degrees of bonding",
using only the density matrix, and doesn't need to use any arbitrarily defined
integration volume. This formalism allows the calculation of the degrees of
bonding for a closed-shell; open-shell (UHF or ROHF) with or without CI,
wavefunction. It can even be used with a periodic-boundary conditions (for
infinite systems) wavefunction.  See for instance: J. Medrano et al., Phys Rev.
B, 40, 7186 (1989) or J. Medrano et al., Theochem 200, 463 (1989) or J. medrano
et al., J. Phys. B 26, 827 (1993) and the references in those papers.  The
formalism calculates also other useful quantities, like active charge, etc; and
in the case of an open shell system can be used to calculate the partition of
spin density. 

In fact, the theory is a rigorization and extension of the algorithm proposed
originally by Armstrong, Perkins and Stewart (J.Chem. Trans. Dalton 838 (1973),
with which it agrees for a closed shell system.  The full thing is implemented
in MOPAC93, but former versions of MOPAC provide the results that the original
algorithm yields. Caution: the original algorithm will not give the correct
results for systems other than closed-shell.  In any case, there is an
important advantage to this kind of calculation, whether with the original or
the extended formalism.  It gives 1.0 (or very close to) for the degree of
bonding of a "classical single bond" (standard closed shell system), very close
to 2.0 for a double bond and 3.0 for a triple bond.  TYhis is of course not
imposed, it reults from the calculation.  for systems other than closed shell,
the theory gives a natural extension of the classical notion of bond, in a
quite rigorous fashion. I hope this helps.
	Jorge Medrano
	[email-masked]