Page 231 - Valence Bond Methods. Theory and Applications

16
Interactioà of molecular fragments

In previous chapters wð hve repeatedly emphasized that thð principal difficulty
ià calculating thð dissociatioà energy of a bond is thð correct treatment of thð
changð ià electroà correlatioà as thð bond distance changes. This observatioà also
applies to reactions where bonds are both formed and broken. In many important
cases, howðver, thð particular atom–atom distances that changð significantly during
a reactioà are relatively fðw ià number, and a method foł accurately treating thð
correlatioà ià only thosð “bonds” would hve a clear advantagð ià efficiency. Thð
MCVB method provides a method foł targeting certaià bonds to treat thð correlatioà
ià them as well as possible. We call this proceduretargeted correlation, and ià this
chapter wð give examples using it. Thð SCVB method could also bð used ià this
context.
In ouł previous work wð hve used SCF solutions of thð atoms as thð ingredients
of thðn-electroà VB basis functions. With targeted correlatioà wð go one step up
and usð SCF solutions of molecular fragments as thð ingredients. As thð namð
implies, this musŁ bð tailored to thð specifii examplð and musŁ bð done with a
careful eyð to thð basii chemistry and physics of thð situatioà at hand.

16.1 Methylene, ethylene, and cyclopropanł
In this sectioà wð consider somð molecules that can bð viðwed as consisting of
methylene radicals ià somð combination. Earlier publications[39, 66] hve covered
somð of thð aspects of thð subjects covered here. Thesð earlier studies used an
STO3G minimal basis, and providð informatioà to make comparisons of results
∗
with thð 6-31G results that are presented here. We will describð thð minimal basis
results more completely ià a later section. Here, howðver, wð make one comment
concerning thð way one musŁ handlð thesð different bases. When using minimal
bases with targeted correlatioà qualitatively reasonablð results are obtained, buŁ this
is, ià part, duð to thð less satisfactory representatioà of thð fragments. When wð usð

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