Fourth age of quantum chemistry
Some 30 years ago, Richards, and later
Schaefer, categorized the development of computational quantum chemistry into
three ages, which was accepted by the community right away. In the first age of quantum chemistry, the
crude quantum mechanical computations were able to provide only qualitative
explanations of relevant experiments, and the agreement between theory and
experiment was within an order of magnitude.
This period lasted up until about the 1950s. In the second age, the tools of quantum
chemistry were developed considerably, the availability of digital computers
shaped the development of the field, and theory started to offer not only
insight into the reasons for the measured properties but also semi-quantitative
results able to help or even shape measurements. Then, the year 1978 was chosen as the start
of the mature, third age of quantum chemistry, whereby theory has become able
to make quantitative predictions and thus challenge (or even overrule)
experiments and/or their interpretations.
It has to be pointed out, however, that within this scheme quantum chemistry
was basically identified as electronic structure theory and thus only the
development of electronic structure theory was considered when the successes of
quantum chemistry were discussed.
Of course, the other important branch of
quantum chemistry besides electronic structure theory deals with the motion of
the nuclei within the molecule, probed usually through high-resolution
molecular spectroscopy or by following chemical reactions. While electronic structure theory has been
quite successful in yielding quantities which can be related, usually at an elementary
level, to experimental observables, truly quantitative agreement with
experiments can only be expected if the motions of the nuclei are also
considered. It is hoped in this context
that we are in, or at least entering, the fourth age of quantum chemistry,
whereby quantum chemistry, now inclusive of both electronic structure and
nuclear motion theories, would quantitatively bridge the gap between
‘effective’, experimental observables and ‘equilibrium’ computed quantities at arbitrary
temperatures of interest and provides results in full quantitative agreement
with the best measurements, help to overrule incorrect measurements, and
substitute experiments when they are too expensive or otherwise impossible to
perform. We may even say tentatively that
the fourth age of quantum chemistry started in one subfield of nuclear motion
theory, in molecular spectroscopy, when we could first demonstrate convincingly
that “third-age” electronic-structure techniques can be used to get
spectroscopic accuracy, defined as 1 cm–1 on average, from purely
first-principles computations for the complete experimentally measured spectra
of all the isotopologues of a polyatomic and polyelectronic molecule, water,
via variational nuclear motion computations.
Related publications:
Richards,
G. Nature 1979, 278, 507.
Schaefer,
H. F. Chimia 1989, 43, 1.
Schaefer, H. F. Science 1986, 231, 1100.
A. G. Császár, G. Czakó, T.
Furtenbacher, and E. Mátyus, An Active Database Approach to Complete
Rotational-Vibrational Spectra of Small Molecules, Ann. Rep. Comp. Chem. 2007,
3,
155-176.
O. L. Polyansky, A. G. Császár,
S. V. Shirin, N. F. Zobov, P. Barletta, J. Tennyson, D. W. Schwenke, and P. J.
Knowles, High-Accuracy Ab Initio
Rotation-Vibration Transitions for Water, Science 2003, 299, 539-542.
A. G. Császár, C. Fábri, T. Szidarovszky, E. Mátyus, T. Furtenbacher,
and G. Czakó, Fourth Age of Quantum Chemistry: Molecules in Motion, Phys. Chem. Chem. Phys. 2012, 14(3), 1085-1106.