There are strong indications that, in the 21st century, computational chemistry will be a prime research tool not only for the basic sciences but also for the life and materials sciences. Recent developments in nanotechnology allow us to detect a layer of single atoms. Researchers are able not only to image but also to manipulate molecules and atoms. It does not take much imagination to realize that before performing such a task on a real system it is much easier and faster to study models on computers. That is the aim of this volume - it provides up-to-date reviews which cover representative areas of computational chemistry. In Chapter 1, Y. Ishikawa and M.J. Vilkas provide a review of multi-reference Moller-Plesset (MR-MP) perturbation theory. Fifteen years ago Roberto Car of Princeton University and Michele Parrinello of Max Planck Institute introduced a method that revolutionized electronic structure calculations for molecules, liquids and solids. Ursula Rothlisberger, a former member of Parrinello's group, reviews the formation of the method in its most common implementations in Chapter 2. In the third chapter, Isaac B. Bersuker describes the general theory of the combined quantum mechanics-molecular mechanics (QM/MM) approach. In Chapter 4, Marcel Allavena and David White present a review of applications of computational chemistry to proton transfer, the primary process for acid-base chemistry on zeolites. Chapter 5 is a review by S. Roszak and J. Leszczynski of recent data on the clusters formed from the charged ion and weakly interacting ligands. The last chapter, contributed by Carlos R. Handy, is devoted to recent developments in the incorporation of continuous wavelet transform analysis into quantum operator theory.