Page 26 - Strategies and Applications in Quantum Chemistry From Molecular Astrophysics to Molecular Engineer

QUANTUM CHEMISTRY: THE NEW FRONTIERS 11
I have touched few items selected in the varied activity of group II not sufficient to give a
balanced appraisal of the evolution and of the prospects of the quantum molecular theory
addressed to interpret chemical facts, but sufficient, I hope, to show that there is here, after
more than thirty years of activity, a noticeable momentum, and that in the foreseeable
future there will be other important progresses.
As a last point I would like to reconsider again a question already examined in relation to
the activities of group I. Is this kind of quantum chemistry able to predict properties, or
molecular species, unknown to experimentalists?

The question is now different from that asked before, because there is no more the demand
of a definitive or fully convincing demonstration. Coming back to the three examples
considered before, it may be said that the discovery of xenon fluorides could have
predicted with theoretical arguments ( and in fact this has been partially done, because
these compounds have been synthesized not by serendipity, but on the basis of theoretical
considerations). The relatively high stability of , and the stability of carbon nanotubes ,
as well as of other more complex structures not yet synthesized, involving knots of
different topology and pseudo-3D lamellar structures, has been already predicted, on the
basis of simple, not definitive, models. The example of polywater shows, on the contrary,
a weakness of the approach. There had been models supporting and describing the
properties of polywater. This activity came at an abrupt end when it was provided
experimental evidence that polywater is a myth. The weakness of interpretative models put
in evidence by this example will be even more critical when put in the context of the
activities of group IV which we shall examine later.
This cautionary remark expressed, we may conclude this section giving a positive answer
to our question. Quantum chemistry, in the version cultivated by group II, represents an
important factor in the growth of chemistry, and constitute one of the cornerstones of
molecular engineering, or similar activities addressed to plan, and to produce, new
substances, new materials endowed with special properties.

4.3. GROUP III

We supplement here the classification proposed by Coulson. The success of quantum
chemistry has given in fact origin to another group, hardly foreseeable in 1959. I am
collecting into group III persons not interested in producing new techniques for the
improvement of in-depth calculations), nor interested in elaborating and checking
interpretative tools, but simply interested in performing and using molecular calculations.
It is a reasonable activity for persons belonging to groups I and II to use, also for extensive
applications, the tools they have elaborated. This activity does not be confused, in my
opinion, with that of members of group III.
There is a variety of motivations for using molecular calculations, some of which are of
interest for the future evolution of theoretical chemistry.
When quantum calculations, at the ab initio and at the semiempirical level, gained foot in
the realm of chemistry, a steadily increasing number of experimentalists began to use
quantum calculations as a supplement in the exposition of their findings. In many case this
was - and still is - nothing more that an ornament, like decorations on a cake. This use of
quantum chemistry has been, in general, harmless, because results in contrast with
experimental evidence have been rarely published, and this production may be considered
now as a sort of advertising for the new-born computational chemistry. A more serious use
of the facilities offered by the computational techniques is done by scientists provided of