Page 15 - Valence Bond Methods. Theory and Applications

xiv
Preface
discussions of structure and reactivity of many chemical substances. It is not my
purpose to touch upon any of this except occasionally. Rather, I will restrict myself
principally to the results and interpretation of the ab initio version of the theory. It
must be admitted that ab initio VB applications are limited to smaller systems, but
we shall stick to this more limited goal. Within what practitioners call ab initio VB
theory there are, in broad terms, two different approaches.
Calculations in which the orbitals used are restricted to being centered on only one atom of

the molecule. They are legitimately called “atomic orbitals”. Treatments of this sort may
have many configurations involving different orbitals. This approach may be considered a
direct descendent of the original Heitler–London work, which is discussed in Chapter 2.
Calculations in which the orbitals range over two or more atomic centers in the molecule.

Although the resulting orbitals are not usually called “molecular orbitals” in this context,
there might be some justification in doing so. Within this group of methods there are
subcategories that will be addressed in the book. Treatments of this sort usually have
relatively few configurations and may be considered descendents of the work of Coulson
and Fisher, which is discussed in Chapter 3.
Each of these two approaches has its enthusiasts and its critics. I have attempted an
even-handed description of them.
At various places in the text there are suggestions for further study to supple-
ment a discussion or to address a question without a currently known answer. The
CRUNCH program package developed by the author and his students is available
1
on the Web for carrying out these studies. This program package was used for all
of the examples in the book with the exception of those in Sections 2.2–2.6.
I wish to thank Jeffrey Mills who read large parts of the manuscript and made
many useful comments with regard to both style and clarity of presentation. Lastly,
I wish to thank all of the students I have had. They did much to contribute to this
subject. As time passes, there is nothing like a group of interested students to keep
one on one’s toes.

Lincoln, Nebraska Gordon A. Gallup
November 2001

1
See http://phy-ggallup.unl.edu/cruncØ