The need for accurate computational procedures to evaluate detailed properties of gas phase chemical reactions is evident when one considers the wealth of information provided by laser, molecular beam and fast How experiments. By stressing ordinary scalar computers to their limiting performance quantum chemistry codes can already provide sufficiently accurate estimates of the stability of several small molecules and of the reactivity of a few elementary processes. However, the accurate characterization of a reactive process, even for small systems, is so demanding in terms of computer resources to make the use of supercomputers having vector and parallel features unavoidable. Sometimes to take full advantage from these features all is needed is a restructure of those parts of the computer code which perform vector and matrix manipulations and a parallel execution of its independent tasks. More often, a deeper restructure has to be carried out. This may involve the problem of choosing a suitable computational strategy or the more radical alternative of changing the theoretical treatment. There are cases, in fact, where theoretical approaches found to be inefficient on a scalar computer exhibit their full computational strength on a supercomputer.

Recent Advances in Electronic Structure Theory and their Influence on the Accuracy of Ab initio Potential Energy Surfaces.- Modern Electronic Structure Calculations: The Accurate Prediction of Spectroscopic Band Origins.- Potential Energy Surfaces of Several Elementary Chemical Reactions.- Calculation and Characterization of Reaction Valleys for Chemical Reactions.- Computed Potential Energy Surfaces for Chemical Reactions.- An Ab initio Study on the Coordination of Formaldehyde, Carbon Dioxide, Dinitrogen and Related Molecules to Iron(0) and Nickel(0) Fragments.- Kinetic Paths from the Hyperspherical Perspective: Ab initio Potential Energy Surface for the O(3P)+H2 Reaction.- Exact Quantum Results for Reactive Scattering Using Hyperspherical (APH) Coordinates.- Computational Strategies and Improvements in the Linear Algebraic Variational Approach to Rearrangement Scattering.- How Variational Methods in Scattering Theory Work.- Quantum Dynamics of Small Systems Using Discrete Variable Representations.- Finite Element Calculations of Scattering Matrices for Atom-Diatom Reactive Collisions. Experiences on an Alliant FX/8.- Investigations with the Finite Element Method. The Collinear H + H2, F + H2 and Ne + H2+ Reactions.- Calculation of Multichannel Eigenvalues and Resonances.- Accurate Determination of Polyatomic Infrared Spectra.- The Calculation of Ro-Vibrational Spectra Using Supercomputers.- Approximate Quantum Techniques for Atom Diatom Reactions.- Approximate Quantum Mechanical Calculations on Molecular Energy Transfer and Predissociation.- Temperature-Dependent Rate Constants for Ion-Dipole Reactions: C+(2P) + HCl(X1?+).- Classical Path Approach to Inelastic and Reactive Scattering.- Intramolecular Energy Transfer in HC and HO Overtone Excited Molecules.-Classical Trajectory Studies of Gas Phase Reaction Dynamics and Kinetics Using Ab initio Potential Energy Surfaces.- Quasiclassical Calculations for Alkali and Alkaline Earth + Hydrogen Halide Chemical Reactions Using Supercomputers.- Dynamics of the Light Atom Transfer Reaction: Cl + HCl ? ClH + Cl.- The Modeling of Complex Gas Phase Reactions: From Expert Systems to Supercomputers.