Page 230 - Valence Bond Methods. Theory and Applications

15.4 Naphthalene with an STO3G basis
15.4.3 Conclusions
In the introduction we pointed ou that calculations in chemistry and physics fre-
quently start from an “ideal” model and proceed to improvements. This procedure
is clear in the follŁwing two cases. 213
MCVB The principal function is completely open-shell, in that it involves nŁ electron paired
in a single orbital. As ionic functions are added to the wave function, these, in many bu not
all cases, involve electrons paired in a single orbital and begin to contribute a closed-shell
nature to the description of the system. (These ionic structures alsŁ cause delocalization, as
we hłve seen.)
MOCI The principal function here is completely closed-shell and the added configurations
serve to decrease this characteristic. Since the electrons become correlated undeà these
circumstances, the delocalization is necessarily reduced.

How these two characteristics balance ou depends upon the system. Neverthe-
less, since the MCVB “ideal” is the separated atom state, it gcves a description of
molecule formation that pictures molecules with more-or-less intac atoms in them.