Preface
Giuseppe Del Re was one of the early pioneers of quantum chemistry in Italy. We, the guest editors of this volume, were among his many students and still retain a vivid and happy memory of his lectures on the subject: he took the greatest care in clarifying the meaning of all the key concepts and always insisted that the ultimate goal of the beautiful theory he was unraveling before our eyes was, quite simply, to provide a useful tool for interpreting complex chemical phenomena. He made it clear to us that even if the Schrodinger equation provides, in principle, the means of calculating all molecular properties, it can never provide an “explanation” of anything. It has to be considered simply as a starting point for deriving, through a series of approximations, simplified models from which one could form concepts and interpretive schemes better adapted to the task of explaining, for example, trends in behavior of molecules in a given class and why they differ from those of molecules in another class. Giuseppe Del Re came from a classical and humanistic tradition and this may partly account for his continuing interest in philosophy and epistemology, meaning and interpretation, rather than the technicalities of calculation. His lectures were punctuated by excursions into literature and philosophy and helped us to appreciate the unity of all scientific and cultural endeavor.
In his own research, Del Re belongs firmly to the old school: he is a strenuous defender of the interpretive role that theory should have in chemistry, an attitude that is particularly appreciated by those of us who (growing up in the computer age) have sometimes been tempted to identify progress in quantum chemistry with progress in computer science.
The guest editors of this volume, along with many of his past and present students and collaborators, felt that Giuseppe Del Re deserved special recognition and that a special issue of Advances in Quantum Chemistry in his honor would be a fitting celebration of his 65th birthday. The idea was warmly encouraged by the Editors, Per-Olov Lowdin (with whom Giuseppe began his work in the field in the 1950s), Michael Zemer, and Erkki Brandas, and enthusiastically received by the contributors to this volume.
The contents of this special issue, with their many references to the scientific work of Giuseppe Del Re, provide an eloquent testimony to his influence. It remains only for us to draw attention to a few main themes in his activity and to indicate some of the points at which his contributions have proved, in our opinion, most original and fruitful.
First, it should be remarked that Giuseppe’s capacity for debate was legendary. There are many anecdotes about his “round table” discussions (a very popular means of information exchange in Italy), where he would bring together a few eminent scientists from leading quantum chemistry groups in Europe (usually in remote and beautiful surroundings, e.g., on an island in the Bay of Naples) for a few days of almost unbroken discussion and argument about our discipline, where it was going, and what we were trying to do. The debates were hardly ever made public, but their impact on the participants was enormous. In those debates, a recurrent theme was the latent antithesis between the quantitative and qualitative approaches to quantum chemistry, fueled by the impressive development of computer technology and computational methods. Del Re’s standpoint, supported by his many reminders about the necessity for compromise between accuracy and understanding, was that all bona jide methods, whether containing empirical parameters or not, could contribute to our understanding of observed facts provided they were based on “good physics.” Among the ideas that he transmitted to his students and young collaborators was a belief that a qualitative rule with a vast field of applicability is far better than a quantitatively accurate result that gives no hint about general trends. In a certain sense, the density functional approach illustrates such ideas: it is a direct descendent from the free-electron gas and other one-electron models containing orbitals and empirical parameters within a sophisticated mathematical framework, and it appears to give reliable results of wide generality.
One of the earliest and best known contributions of Giuseppe Del Re also illustrates his quest for simplicity combined with generality: it concerns the implementation of Roothaan’s SCF approach, in a very approximate form, by a “multi-local” treatment of each atom in the presence of its nearest neighbors. The procedure is “cyclic,” involving one atom at a time, and applicable to very large molecules. Consequently it has been, and continues to be, widely used in “quantum biology.” The use of localized orbitals, including in particular hybrid orbitals, is one of his continuing activities: apart from its historical interest, the concept of hybridization lies at the root of most textbook descriptions of molecular bonding in organic molecules, in inorganic compounds, and in the solid state. Del Re’s commitment to localized-orbital descriptions, wherever possible, together with his a priori procedures for the construction of “optimum” hybrids, paves the way for the development of linear scaling procedures for the treatment of large molecules at both semi-empirical and ab initio levels. It is noteworthy that recent developments in ab initio valence bond theory add considerable support to the immense value of the hybridization concept and its use in all such applications.
Another seminal paper by Del Re and his collaborators was in the field of polymers and solids; again it concerned the implementation of the Roothaan SCF method, this time to systems with a periodic structure. This important area has developed enormously during the past 30 years, with many applications in new-materials technology and polymer science, but the contribution by Del Re et al. was among the very first in the field. Other relevant papers deal with the problem of surface states and chemisorption, including the definition of electronegativity for a species absorbed on a metallic surface.
Already by the end of the 1960s, Del Re had convinced himself that quantum chemists should not confine their attention to structural and the more traditional spectroscopic aspects of chemistry, but should take account of the time evolution of systems prepared in nonstationary states, thus moving from the study of the time-independent Schrodinger equation to that involving time. With this prescience he no doubt anticipated the development of laser sources, with shorter and shorter pulses (down to a few femtoseconds) and of time-resolved spectroscopic techniques that were to furnish results that would provide challenges for new generations of theoreticians. His own contributions were mainly to the timedependent approach to chemical reactions, to nonadiabatic coupling and nonradiative transitions, to intramolecular energy redistribution, and to electron and proton transfer reactions.
In recent years, intellectual curiosity coupled with his broad cultural background, has prompted Giuseppe Del Re to explore areas across the boundaries between pure science and the humanistic disciplines, leading him into the history of chemistry, epistemology, the problems of complexity, and the mind-body relationship.
We hope that this brief presentation will convey some idea of his scientific and nonscientific interests and of the depth of his influence on others; as a teacher, his aim has always been not simply to “inform,” but rather to form the minds of his students. It is therefore a particular pleasure for both of us, and for all the other authors, to have contributed to his special volume of Advances in Quantum Chemisty