Neither Physics nor Chemistry: A History of Quantum Chemistry

Introduction

Introduction Although it is relatively easy to relate what something is not, it is always challenging to be clear about what something is. The first part of the title of our book clearly delineates what quantum chemistry is not. The rest of the title is a promise to tell what this discipline is and how it developed. One year before the year we chose to end our narrative—with the Conference on Computational Support for Theoretical Chemistry in 1970—at a symposium on the “Fifty Years of Valence,” Charles Alfred Coulson, one of the protagonists of our story and Rouse Ball Professor of Applied Mathematics at the University of Oxford at the time, talked of chemistry as a discipline that is concerned with explanation and cultivates a sense of understanding. “Its concepts operate at an appropriate depth and are designed for the kind of explanation required and given” (Coulson 1970, 287). He noted that when the level of inquiry deepens, then a number of older concepts are no longer relevant. And then, Coulson emphatically declared that one of the primary tasks of the chemists during the initial stage in the development of quantum chemistry was to escape from the thought forms of the physicists (Coulson 1970, 259, emphasis ours). Indeed. Among the many and, at times, insurmountable barriers during the development of quantum chemistry, perhaps the one hurdle that was the most incapacitating was the prospect of problems of (self)identity the new subdiscipline would have: It appeared that whatever was done to lead to the establishment of quantum chemistry as a subdiscipline in chemistry would, in effect, be indistinguishable from whatever was needed to establish it as a subdiscipline of physics! Hence, escaping the thought forms of the physicists was a strategic choice in developing the culture of the new subdiscipline and in articulating its practices—not consciously pursued by all, but, surely, in the minds of those whose work eventually established the subdiscipline. And Coulson, more than anyone else, turned out to be particularly sensitive to the almost imperceptible borderline between physics and chemistry when one decided to “deepen the level of inquiry.” Nearly at the same time, the Swedish quantum chemist Per-Olov Löwdin, professor of quantum chemistry at the University of Uppsala and the founder of the International 2 Introduction Journal of Quantum Chemistry in 1967, wrote in the editorial of the first issue that quantum chemistry “uses physical and chemical experience, deep going mathematical analysis and high speed electronic computers to achieve its results.” He acknowledged that quantum mechanics was offering a framework for the unification of all the natural sciences—including biology. And, as for quantum chemistry, he emphasized that it was a young field “which falls between the historically developed areas of mathematics , physics, chemistry, and biology” (Löwdin 1967, 1). Both Coulson and Löwdin, though they were clear about the kinds of problems quantum chemistry tackled, were, somewhat uncertain as to the signifying characteristics of its culture and practices. Coulson tells us that chemistry explains and gives insight and a sense of understanding—but this is the case in a host of other disciplines. We are told that its concepts operate at an appropriate depth and they cater for the kind of explanation we seek—again, something all too common in many other disciplines . It is noted that these concepts are no longer relevant when our inquiry deepens—again, as it happens in many other disciplines. Two outstanding quantum chemists such as Coulson and Löwdin, despite their thoughtful comments about the status of quantum chemistry, were, in effect, expressing their uneasiness when it came to delineate the methodological, philosophical, and disciplinary boundaries of quantum chemistry, echoing what was discussed in meetings, what was stated in papers, what was implied in textbooks, throughout the four decades since the 1927 paper of Walter Heitler and Fritz London who showed in no uncertain terms that the covalent bond—a kind of mystery within the classical framework—could be mathematically tackled and physically understood by using the recently formulated quantum mechanics. In a way, our narrative is the unfolding of this uneasiness while at the same time it displays the variety of strands whose synthesis gave rise to quantum chemistry: the different methodological traditions that came to the fore, the decisions of the leaders of each tradition to consolidate a framework of practices, the rhetorical strategies and the processes of legitimization, the role of textbooks, journals, and conferences in building the relevant scientific...