At the time when increasing numbers of chemists are being attracted by the fascination of supposedly easy computing and associated colourful imaging, this book appears as a counterpoint. The first part focuses on fundamental concepts of quantum chemistry, covering MCSCF theory, perturbation treatments, basis set developments, density matrices, wave function instabilities to correlation effects, and momentum space theory. The second part is devoted to more practical studies, ranging from the characterisation of exotic interstellar molecules, the accurate determination of spectroscopic constants, excited states structures and EPR parameters through photochemical and charge-transfer processes, cluster chemistry and fullerenes....