Page 17 - Strategies and Applications in Quantum Chemistry From Molecular Astrophysics to Molecular Engineer

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J. TOMASI
the meaning of theoretical chemistry which now may be defined as the discipline studying
molecules with quantum mechanical methods.
The progress in science does not proceed with a steady pace. Periods of quantitative
growths, often rich of results, begin, and end, with sudden changes which gives rise to a
quantitative turn in the research methods.
In my opinion, the last qualitative change in theoretical chemistry corresponds to the
introduction of computers in chemistry. A conventional date for the beginning of this last
period may be indicated in the Boulder Conference of 1959 [2], i.e. more than thirty years
ago. The use of more and more large and efficient computers has shifted the attention of
theoretical chemists to an extensive use of quantum mechanical calculations.

Quantum mechanics was the dominant theory in chemistry even before the advent of
electronic computers. The conventional date for the beginning of this period may be fixed
at 1927 with the publications of the Heitler and London paper on hydrogen molecule [3].
The growth of theoretical chemistry (or better, theoretical quantum chemistry) between
1930 and 1960 (thirty years, again, as for the last period) has followed a research
programme different from that accepted in the most recent period.

We shall return later on this difference of approach. Before the advent of quantum
mechanics theoretical chemistry was influenced by the lack of a comprehensive theory for
matter at the microscopic level. In the preceding thirty years, i.e. from the beginning of this
century, there has been an evolution of the main line of research, based on the adoption of
approaches (paradigms) derived from physics with a progressive shift from an alternative
approach, based on chemical concepts, elaborated during the last part of the preceding
century by structural chemists. The physical approach has given much emphasis to the
molecule, considered as a physical entity, the properties of which are sufficient to interpret,
and to predict, the chemical behaviour of matter.
According to this partisan view of the evolution of theoretical chemistry we draw the
impression of a choice, in which the single molecules represent the basic unit of
investigation, the quantum theory provide the theoretical basis, and computer calculations
the final step. The three periods of growth are, in reality related, and the "sudden" changes
in between do not corresponds to "revolutions" in according to the meaning this word has
in the Kuhn's analysis [4].
Just at the closing speech of the event we have chosen as indicative of the beginning of the
last period, the Boulder Conference of 1959, C.A. Coulson [5] expressed the
preoccupation that the new era of theoretical chemistry, so bright of exciting promises,
would also lead to a splitting of the discipline into two (or to be more precise, three)
separate domains, each having its own set of paradigms, and not paying much attention to
the evolution of the other domains.
According to the Coulson words, the exponents of group I were committed to "in-depth
computing" and "prepared to abandon all the chemical concepts and simple pictorial quality
in their results" "in order to achieve complete accuracy"; while "the exponent of group II
argue that chemistry is an experimental subject, whose results are built into a pattern
around quite elementary concepts". The third group was at that moment (1959) more a
hope than a reality; the "spreading of quantum chemistry to biology": "Group I exponents
will throw up their hands in horror at such attempts", "group II members will mistrust the
complete neglect of many terms which are known to be large", but "the prizes are
immense": "there is much experience possessed by professional biologists which could be
linked with the deeper levels of interpretation associated with quantum chemistry", even if
"biological systems are much more perverse than any laboratory chemical system".