The present volume gathers a series of selected and updated contributions presented at the International Symposium on Applied Theoretical Chemistry held in Havana, Cuba, July 2-6, 1990. This Symposium was intended to illustrate current applications of Theoretical Chemistry in different fields of Physical Chemistry. Theoretical Chemistry has become a powerful tool of investigation in all areas of Chemistry, Biochemistry, and Physical Chemistry. The plenary lectures given in the Symposium were classified into four topics: Atom-Surface Interactions, Chemical Reaction Mechanisms, Molecular Structure and Properties, and Molecular Spectroscopy. We retain the same division in this volume. Over 60 scientists from Cuba, Finland, France, Germany, Great-Britain, Hungary, Italy, Spain, Sweden, USA, USSR, and Venezuela participated in the Conference. Twenty plenary lectures were given by distinguished members of the international scientific community. Furthermore, a large number of posters were presented by younger experts in various fields of Theoretical Chemistry. This International Symposium was organized by the Faculty of Chemistry of the University of Havana and the Cuban Chemical Society. It was an opportunity to bring together in Havana several outstanding scientists from various countries of the world. Havana is worldwide renown for its wonderful climate, the hospitality of its inhabitants, and the proximity of beautiful touring resorts.