Page 20 - Strategies and Applications in Quantum Chemistry From Molecular Astrophysics to Molecular Engineer

QUANTUM CHEMISTRY: THE NEW FRONTIERS 5

are not complete and often in competition (A given phenomenon may be described in
different ways, using different concepts, and invoking different "causes").

3) Explanation. This last step aims at reaching a fuller comprehension of the
phenomenon. Contrasting descriptions must find here a synthesis. A satisfying
explanation cannot be reached by examining the descriptions of the report of a single
model, but must consider a whole set of models. As members of the chemical
community we also require that the explanation so obtained applies to the "objects"
of the real word of which the model is a schematic representation.

A partition of the process of understanding a complex phenomenon into three steps has
been supported and justified by Runcimann [12] for social sciences. The definitions done
by Runcimann cannot be directly translated into our field, nor the names he selected for the
sequence of levels, but his scheme, presented here in a modified form, gives a contribution
to appreciate the strategy and the impact of specific works of research.

4. The different facets of quantum chemistry
4.1. GROUP I

The definitions given by Coulson to quantum chemists belonging to this group (electronic
computors, or ab initio-ists) is surely outdated. Every quantum chemists is now an
"electronic computer" and the difference between ab-initioists and non ab-initioists is rather
feeble.
There is a large variety of motivations and strategies for persons and works collected here
under this heading. The effort of making more efficient the computational algorithms,
extending thus the area of material and physical models for which the report becomes
satisfactory and quite exhaustive, has produced results of paramount importance.

The good success of these efforts has greatly improved the status of quantum chemistry in
the scientific community.Quantum chemistry is now one respectable branch of chemistry,
like organic synthesis or molecular spectroscopy, because their practitioners have shown
that high-level quantum calculations are not confined to models composed by 2-10
electrons, and that the information thus gained is valuable and comparable to that
obtainable with the aid of other methods. This achievement could be considered of
secondary interest ("well, there is another technique which confirms our evidences"), but
actually has had a great impact on the evolution of chemical thinking and teaching, suffice
to compare textbooks of chemistry ante 1960 with the present ones.

The future evolution of chemistry will be more and more based on theoretical concepts,
and we have to ascribe to "in-depth computors" the merit of this evolution, even if quite
probably the most significant progresses will not directly derive from very accurate
calculations.

This line of research has not lost his momentum. One of the reasons is the continuing
progress in the computer hardware and software. Methods and algorithms are, and will be,
continuously updated to exploit new features made available by computer science, as for
example the parallel architectures, or the neuronal networks, to mention things at present of
widespread interest, or even conceptually less significant improvements, as the increase of
fast memory in commercial computers. Computer quantum chemistry is not a mere
recipient of progresses in computer science. Many progresses in the software comes from