Fifty Years of Quantum Chemistry

An assessment of some aspects of the impact of quantum mechanics on chemistry will be attempted, with emphasis on the different ways it has had an important influence. These include a change in the chemists' vocabulary, his way of looking at molecules and reactions, the instruments and methods he uses, the psychological satisfaction he gets from his subject, the procedures he uses to describe and to rationalize his results, as well as direct, partly predictive theories and numerical calculations. The introduction and improvement of the electronic computer played a major role but also diverted talent away from the development of new approaches. A number of other questions about quantum chemistry will be raised and personal answers offered. Fifty years should be long enough for a new approach to have a major impact on a science, and there is no doubt about the magnitude of the impact of quantum chemistry. Quantum mechanics as applied to chemistry has an astonishing history. Only three years after its first formulation in 1926 Dirac made his famous statement' "The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known. . .", yet at that time not a single accurate chemical i.e. molecularapplication of quantum mechanics had been made. Nevertheless, Dirac's views have been shared ever since by most theoretical chemists, despite the fact that even today with mammoth computers not even dreamed of in 1929, only a few properties of a handful of the most trivial molecules can be calculated with an accuracy approaching experiment. How can one say that scientists lack faith! I wish now to discuss the impact of quantum mechanics on chemistry by asking—and then attempting to answer— a series of questions. Some of the answers will be personal judgments with which not everyone will agree. The first question is: Is quantum mechanics correct? The answer is: surely not. Relativity is not properly taken into account, and of course the physicist has gone beyond ordinary elementary quantum mechanics in order to deal with high energy phenomena, appearing and disappearing particles, quantized radiation fields, etc. Is ordinary quantum mechanics good enough for chemistry? For the lighter elements and most phenomena of chemistry, I would vote "yes". For the heavier elements, there still seem to be unresolved problems with relativistic effects. Table 1 shows some estimated corrections.2 These suggest that not all is well with heavier elements, although recent non-relativistic calculations on lead oxide3 have been interpreted as supporting the hypothesis that such calculations can be used. Why should we believe that quantum mechanics is in Table 1. Relativistic corrections for the ground state of some atoms.2 (Hartrees, 1 Hartree = 6275 kcal)