Chemistry as an exact science.- Computational bottlenecks in molecular orbital calculations.- Variational transition state theory calculations of concerted hydrogen atom tunneling in water clusters and formaldehyde / water clusters.- Double many-body expansion potential energy surface for O4(3A), dynamics of the O(3P) + O3(1A1) reaction, and second virial coefficients of molecular oxygen.- The self-consistent reaction field model for molecular computations in solution.- New symmetry theorems and similarity rules for transition structures.- A topological analysis of macromolecular folding patterns.- Molecular mechanics.- Predicting the three-dimensional structure of proteins by homology-based model building.- Understanding chemical reactivity through the intersecting-state model.- The states of an electron pair and photochemical reactivity.- Ab-initio modelling of chemical reactivity using MC-SCF and VB methods.- The supra-supra mechanism of forbidden and allowed cycloaddition reactions: an analysis a VB model.- Excited state proton transfer reactions.- An exploratory study to correlate experimental and theoretical acidities of organic molecules.- Molecules with "volcanic" ground hypersurfaces. Structure, stability and energetics.- Molecular hydrogen as a ligand in transition metal complexes.- Molecular orbital studies of reductive elimination reactions.- Laboratory projects in computational organic chemistry.

The papers in this volume were presented at the NATO Advanced Study Institute held in Porto Novo, Portugal, August 26 - September 8, 1990. The Institute has been able to cover a wide spectrum of the Theoretical and Computational Models for organic molecules and organic reactions, ranging from the ab initio to the more empirical approaches, in the tradition established in the previous Institutes at S. Feliu de Guixols (Spain) and Altinoluk (Turkey). The continuity with this work was achieved by inviting half of the lecturers present in those meetings. But other important subjects were also covered at Porto Novo by new lecturers, both from universities and the industry. Molecular Mechanics, Protein Structure and Unidimensional Models were introduced by the first time. The concept of building on the expertise already acquired and available, both in terms of methods and contents, to develop in new directions, was appreciated by participants and lecturers. The Institute first considered the fundamentals of molecular orbital computations and ab initio methods and the construction of Potential Energy Surfaces. These subjects were further explored in several applications related with optimization of equilibrium geometries and transition structures. Practical examples were studied in Tutorial sessions and solved in the computational projects making use of the Gaussian 88 and Gaussian 90 programs. Empirical models can be complementary to the quantum-mechanical ones in equilibrium geometry optimizations.