Book excerpt: Computational quantum chemistry was born in the mid 1960s, and had by the early 1980s achieved considerable status as a structural tool within chemistry. The field has now developed to the point where it has its own journals. However, a major change is taking place in that most consumers of computational quantum chemistry are now experimentalists, who want answers to questions of the type "What if.....?" This change has come about because of the dramatic fall in computer hardware costs, the ready availability of large molecular structure packages and the international collaboration between quantum chemists on a scale rarely witnessed in science. This book aims to show what can be done by computational chemistry, and what kind of reliance might be placed on the predictions. The vast majority of investigations are made at the 'ab initio self consistent field' level, and the results of such calculations occupy a prominent role in this book. However, the user has to be aware of the limitations of this model, and the effects upon electron correlation, etc are discussed. Anyone who is contemplating making use of the techniques of computational quantum chemistry to rationalise or predict chemical behaviour will find this unique book of tremendous use.