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.
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