Preface

This book has been written by chemists for chemists. In particular, it has not been written by genuine theoretical chemists but by chemists who are primarily interested in solving chemical problems and in using computational methods for addressing the many exciting questions that arise in modern chemistry. This is important to realize right from the start because our background of course determined how we approached this project. Density functional theory is a fairly recent player in the computational chemistry arena. WK, the senior author of this book remembers very well his first encounter with this new approach to tackle electronic structure problems. It was only some ten years back, when he got a paper to review for the Journal of Chemical Physics where the authors employed this method for solving some chemical problems. He had a pretty hard time to understand what the authors really did and how much the results were worth, because the paper used a language so different from conventional wave function based ab initio theory that he was used to. A few years later we became interested in transition-metal chemistry, the reactivity of coordinatively unsaturated open-shell species in mind. During a stay with Margareta Blomberg and Per Siegbahn at the University of Stockholm, leading researchers in this field then already for a decade, MCH was supposed to learn the tricks essential to cope with the application of highly correlated multireference wave function based methods to tackle such systems. So he did - yet, what he took home was the feeling that our problems could not be solved for the next decade with this methodology, but that there might be something to learn about density functional theory (DFT) instead. It did not take long and DFT became the major computational workhorse in our group. We share this kind of experience with many fellow computational chemists around the globe. Starting from the late eighties and early nineties approximate density functional theory enjoyed a meteoric rise in computational chemistry, a success story without precedent in this area. In the Figure below we show the number of publications where the phrases 'DFT' or 'density functional theory' appear in the title or abstract from a Chemical Abstracts search covering the years from 1990 to 1999. The graph speaks for itself.

This stunning progress was mainly fueled by the development of new functionals - gradient-corrected functionals and most notably hybrid functionals such as B3LYP - which cured many of the deficiencies that had plagued the major model functional used back then, i. e., the local density approximation. Their subsequent implementation in the popular quantum chemistry codes additionally catalyzed this process, which is steadily gaining momentum. The most visible documentation that computational methods in general and density functional theory in particular finally lost their 'new kid on the block' image is the award of the 1998 Noble Prize in chemistry to two exceptional protagonists of this genre, John Pople and Walter Kohn.

Many experimental chemists use sophisticated spectroscopic techniques on a regular basis, even though they are not experts in the field, and probably never need to be. In a similar manner, more and more chemists start to use approximate density functional theory and take advantage of black box implementations in modern programs without caring too much about the theoretical foundations and - more critically - limitations of the method. In the case of spectroscopy, this partial unawareness is probably just due to a lack of time or motivation since almost any level of education required seems to be well covered by textbooks. In computational chemistry, however, the lack of digestible sources tailored for the needs of chemists is serious. Everyone trying to supplement a course in computational chemistry with pointers to the literature well suited for amateurs in density functional theory has probably had this experience. Certainly, there is a vast and fast growing literature on density functional theory including many review articles, monographs, books containing collections of high-level contributions and also text books. Indeed, some of these were very influential in advancing density functional theory in chemistry and we just mention what is probably the most prominent example, namely Parr's and Yang's 'Density-Functional Theory of Atoms and Molecules' which appeared in 1989, just when density functional theory started to lift off. Still, many of these are either addressing primarily the physics community or present only specific aspects of the theory. What is not available is a text book, something like Tim Clark's 'A Handbook of Computational Chemistry', which takes a chemist, who is interested but new to the field, by the hand and guides him or her through basic theoretical and related technical aspects at an easy to understand level. This is precisely the gap we are attempting to fill with the present book. Our main motivation to embark on the endeavor of this project was to provide the many users of standard codes with the kind of background knowledge necessary to master the many possibilities and to critically assess the quality obtained from such applications. Consequently, we are neither concentrating on all the important theoretical difficulties still related to density functional theory nor do we attempt to exhaustively review all the literature of important applications. Intentionally we sacrifice the purists' theoretical standpoint and a broad coverage of fields of applications in favor of a pragmatic point of view. However, we did our best to include as many theoretical aspects and relevant examples from the literature as possible to encourage the interested readers to catch up with the progress in this rapidly developing field. In collecting the references we tried to be as up-to-date as possible, with the consequence that older studies are not always cited but can be traced back through the more recent investigations included in the bibliography. The literature was covered through the fall of 1999.

However, due to the huge amount of relevant papers appearing in a large variety of journals, certainly not all papers that should have come to our attention actually did and we apologize at this point to anyone whose contribution we might have missed. One more point: we have written this book dwelling from our own background. Hence, the subjects covered in this book, particularly in the second part, mirror to some extent the areas of interest of the authors. As a consequence, some chemically relevant domains of density functional theory are not mentioned in the following chapters. We want to make clear that this does not imply that we assign a reduced importance to these fields, rather it reflects our own lack of experience in these areas. The reader will, for example, search in vain for an exposition of density functional based ab initio molecular dynamics (Car-Parrinello) methods, for an assessment of the use of DFT as a basis for qualitative models such as soft- and hardness or Fukui functions, an introduction into the treatment of solvent effects or the rapidly growing field of combining density functional methods with empirical force fields, i. e., QM/MM hybrid techniques and probably many more areas.

The book is organized as follows. In the first part, consisting of Chapters 1 through 7, we give a systematic introduction to the theoretical background and the technical aspects of density functional theory. Even though we have attempted to give a mostly self-contained exposition, we assume the reader has at least some basic knowledge of molecular quantum mechanics and the related mathematical concepts. The second part, Chapters 8 to 13 presents a careful evaluation of the predictive power that can be expected from today's density functional techniques for important atomic and molecular properties as well as examples of some selected areas of application. Of course, also the selection of these examples was governed by our own preferences and cannot cover all important areas where density functional methods are being successfully applied. The main thrust here is to convey a general feeling about the versatility but also the limitations of current density functional theory.

For any comments, hints, corrections, or questions, or to receive a list of misprints and corrections please drop a message at DFT-Guide@chemie.uni-marburg.de.

Many colleagues and friends contributed important input at various stages of the preparation of this book, by making available preprints prior to publication, by discussions about several subjects over the internet, or by critically reading parts of the manuscript. In particular we express our thanks to V. Barone, M. Bühl, C. J. Cramer, A. Fiedler, M. Filatov, F. Haase, J. N. Harvey, V. G. Malkin, P. Nachtigall, G. Schreckenbach, D. Schröder, G. E. Scuseria, Philipp Spuhler, M. Vener, and R. Windiks. Further, we would like to thank Margareta Blomberg and Per Siegbahn for their warm hospitality and patience as open minded experts and their early inspiring encouragement to explore the pragmatic alternatives to rigorous conventional ab initio theory. WK also wants to thank his former and present diploma and doctoral students who helped to clarify many of the concepts by asking challenging questions and always created a stimulating atmosphere. In particular we are grateful to A. Pfletschinger and N. Sändig for performing some of the calculations used in this book. Brian Yates went through the exercise of reading the whole manuscript and helped to...