Subject: HF and EC bond lengths

From: Frank Jensen <[email-masked]>
Date: Tue, 18 Nov 1997 12:28:53 +0100 (MET)

Dear All,

	For 'normal' organic molecules the general trend
is that Hartree-Fock geometries have bond lengths which are
too short (at least when sufficiently large basis sets are used).
Addition of electron correlation increases the bond lengths,
and MP2 with a large basis set is often surprisingly close
to the experimental value. This behavior is often rationalized
by arguing that the Hartree-Fock wave function has the
wrong dissociation limit, and by the fact that electron
correlation in general increases as a function of bond length. 

	For metal coordinated species (e.g. ferocene), however,
the observed trend is normally the opposite, i.e. Hartree-Fock
geometries have too long bond lengths and electron correlation
makes them shorter. MP2 normally is quite poor in these cases,
and overshoots the correlation effect significantly. Coupled
cluster method (e.g. CCSD) give quite good geometries, as does
DFT.

	Does anyone have a good rational for this change
in behavior? The arguments for the 'normal' molecules should
be equally valid for the metal coordinates species, but the
observed trend is the opposite.

	Frank