Page 28 - Strategies and Applications in Quantum Chemistry From Molecular Astrophysics to Molecular Engineer

QUANTUM CHEMISTRY: THE NEW FRONTIERS 13
Computers of group III (let me use again this disparaging definition; it would be clear now
that my personal position is far from being disparaging) are shifting their interests to
problems of ever increasing complexity, because this is the evolution of chemistry, and are
now affording problems hardly treatable with canonical procedures elaborated for
molecules containing a moderate number of atoms. These problems represent a new
challenge to the theory, and this is the field of investigation of the last group in our
classification.

4.4. GROUP IV
Coulson signalled the possible formation of a separate group related to "the spreading of
quantum chemistry into biology". This prediction is now a reality and Quantum Biology is
an important branch of Quantum Chemistry [27], cultivated by members of all the
preceding groups. The contribution of group I via the elaboration of new formalisms as
well as via the elaboration of more powerful computational techniques constitutes the basic
layout; concepts and interpretations provided by group II find here an exciting field of
application (and a challenge to refine and to extend the methods); the computational
enthusiasm of group III with its combination of different approaches is especially
addressed to these problems.

I prefer do not consider scientists working in quantum biology as a separate group, but
rather to collect a sizable part of their activity into a more general group, characterized by
the presence in their problems of a large number of degrees of freedom. We could collect
here all the problems regarding matter in condensed phases, from real gases to perfect
crystals. In this very large body of systems - and of phenomena - many are not sensibly
affected by the increase of the degrees of freedom, and the traditional approaches are still
sufficient.

More interesting is the consideration of cases in which the traditional approach is ill at ease.
The theory of chemical bonding is not profoundly affected (special cases apart) from the
extension of the number of degrees of freedom. Clementi rightly pointed out that from the
point of view of quantum mechanical calculations there are no "too large" systems: the
portion of space including the matter exhibiting a non vanishing interaction with a localized
subunits (e.g. an atom or a bond) may be defined in terms of a sphere, with a radius
not extremely large. Nowadays our computational tools are able to fill almost completely
this sphere with interacting matter (electrons, nuclei) and to describe the interactions at a
reasonable (and steadily increasing) level of accuracy. This concept may be introduced into
our definition of models for quantum chemistry: there will be an overlap of material sub-
models, with defined physical interactions, and the whole problem is then reduced to the
specification of the mathematical model able to deal with the couplings among subunits. A
formidable problem is thus reduced to a more manageable form.

A report on the electronic structure of a large molecule at a given geometry is however the
first in a long sequence of steps. Even the next step, the recognition of the features of the
potential energy hypersurface presents formidable problems, well known to members of
group III who study conformational properties of large molecules. There are now
expedient ways to overcome (in part) the difficulties of this specific problem, but
analogous questions rise again at a higher level of investigation, when the "large
molecules" are involved in chemical reactions. This last problem is present, and perhaps
more evident, in the study of chemical reactions involving "small" molecules in condensed
media.