At the American Chemical Society meeting in Philadelphia, Pennsylvania, U.S.A., a symposium was organized entitled, "Comparison of Ab Initio Quantum Chemistry with Experiment: State-of-the-Art." The intent of the symposium was to bring together forefront experimen talists, who perform the types of clean, penetrating experiments that are amenable to thorough theoretical analysis, with inventive theore ticians who have developed high accuracy ab initio methods that are capable of competing favorably with experiment, to assess the current applicability of theoretical methods in chemistry. Contributions from many of those speakers (see Appendix A) plus others selected for their expertise in the subject are contained in this volume. Such a book is especially timely, since with the recent develop ment of new, more accurate and powerful ab initio methods coupled with the exceptional progress achieved in computational equipment, ab initio quantum chemistry is now often able to offer a third voice to resolve experimental discrepancies, assist essentially in the interpre tation of experiments, and frequently, provide quantitatively accurate results for molecular properties that are not available from experiment.

At the American Chemical Society meeting in Philadelphia, Pennsylvania, U.S.A., a symposium was organized entitled, "Comparison of Ab Initio Quantum Chemistry with Experiment: State-of-the-Art." The intent of the symposium was to bring together forefront experimen talists, who perform the types of clean, penetrating experiments that are amenable to thorough theoretical analysis, with inventive theore ticians who have developed high accuracy ab initio methods that are capable of competing favorably with experiment, to assess the current applicability of theoretical methods in chemistry. Contributions from many of those speakers (see Appendix A) plus others selected for their expertise in the subject are contained in this volume. Such a book is especially timely, since with the recent develop ment of new, more accurate and powerful ab initio methods coupled with the exceptional progress achieved in computational equipment, ab initio quantum chemistry is now often able to offer a third voice to resolve experimental discrepancies, assist essentially in the interpre tation of experiments, and frequently, provide quantitatively accurate results for molecular properties that are not available from experiment.

The principal focus of this volume is to illustrate the level of accuracy currently achievable by ab initio quantum chemical calculations. While new developments in theory are discussed to some extent, the major emphasis is on a comparison of calculated properties with experiment. This focus is similar to the one taken in a book, Comparison of Ab Initio Quantum Chemistry with Experiment for Small Molecules, edited by Rodney Bartlett (Reidel, 1984). However, the phenomenal improvement in both theoretical methods and computer architecture have made it possible to obtain accurate results for rather large molecular systems. This is perhaps best illustrated in this volume by the chapter entitled `Spectroscopy of Large Organic Molecules' by Bjorn Roos and coworkers. For example, the electronic spectra of the nucleic acid base monomer structures shown on the front cover have been obtained using a fully correlated ab initio study. For researchers, teachers and students in chemistry and physics.

Until recently quantum chemical ab initio calculations were re stricted to atoms and very small molecules. As late as in 1960 Allen l and Karo stated : "Almost all of our ab initio experience derives from diatomic LCAO calculations ••• N and we have found in the litera ture "approximately eighty calculations, three-fourths of which are for diatomic molecules ••• There are approximately twenty ab initio calculations for molecules with more than two atoms, but there is a decided dividing line between the existing diatomic and polyatomic wave functions. Confidence in the satisfactory evaluation of the many -center two-electron integrals is very much less than for the diatom ic case". Among the noted twenty calculations, SiH was the largest 4 molecule treated. In most cases a minimal basis set was used and the many-center two-electron integrals were calculated in an approximate way. Under these circumstances the ab initio calculations could hard ly provide useful chemical information. It is therefore no wonder that the dominating role in the field of chemical applications was played by semiempirical and empirical methods. The situation changed essentially in the next decade. The problem of many-center integrals was solved, efficient and sophisticated computer programs were devel oped, basis sets suitable for a given type of problem were suggested, and, meanwhile, a considerable amount of results has been accumulated which serve as a valuable comparative material. The progress was of course inseparable from the development and availability of computers.