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Until recently quantum chemical ab initio calculations were re
stricted to atoms and very small molecules. As late as in 1960
Allen l and Karo stated : "Almost all of our ab initio experience
derives from diatomic LCAO calculations *** N and we have found in
the litera ture "approximately eighty calculations, three-fourths
of which are for diatomic molecules *** There are approximately
twenty ab initio calculations for molecules with more than two
atoms, but there is a decided dividing line between the existing
diatomic and polyatomic wave functions. Confidence in the
satisfactory evaluation of the many -center two-electron integrals
is very much less than for the diatom ic case". Among the noted
twenty calculations, SiH was the largest 4 molecule treated. In
most cases a minimal basis set was used and the many-center
two-electron integrals were calculated in an approximate way. Under
these circumstances the ab initio calculations could hard ly
provide useful chemical information. It is therefore no wonder that
the dominating role in the field of chemical applications was
played by semiempirical and empirical methods. The situation
changed essentially in the next decade. The problem of many-center
integrals was solved, efficient and sophisticated computer programs
were devel oped, basis sets suitable for a given type of problem
were suggested, and, meanwhile, a considerable amount of results
has been accumulated which serve as a valuable comparative
material. The progress was of course inseparable from the
development and availability of computers.
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