Anderson has made key contributions in several areas of chemistry and physics. The main areas of impact are: reaction kinetics and molecular dynamics, the 'rare-event' approach to chemical reactions, Quantum Monte Carlo methods, Monte Carlo simulation of radiative processes, and direct Monte Carlo simulation of reaction systems. Anderson’s first contributions were experimental and theoretical in the area of nozzle-source molecular beams and the free jet fuels and skimmers for generating such beams. This research contributed to success in generating molecular beams of high energy and narrow velocity distributions. Anderson‘s experiments with supersonic beams for the reaction HI + HI → H2 + I2 led him to early studies using classical trajectory methods. He carried out the first calculations of the F-H-H system with a study of the energy requirements for the reaction H + HF → H2 + F and followed this work with calculations for F + H2 → HF + H, a reaction basic to the understanding of molecular dynamics. Trajectory calculations for the HI + HI reaction, a rare event, led to his work on predicting rare events in molecular dynamics by sampling trajectories crossing a surface in phase space. Initially called “variational theory of reaction rate” by James C. Keck, it has since 1973 often been called “the reactive flux method.” Anderson extended Keck’s original method and defended it against a number of critics. The earliest applications were to three- and four-body reactions, but it has been extended to reactions in solution, to condensed matter, to protein folding, and most recently to enzyme-catalyzed reactions. Anderson pioneered the development of the quantum Monte Carlo method of simulating the Schrödinger equation. His 1975-76 papers were the first to describe applications of random walk methods to polyatomic systems and many-electron systems. Today, QMC methods are often the methods of choice for high accuracy for a range of systems: small and large molecules, molecules in solution, electron gas, clusters, solid materials, vibrating molecules, and many others. Anderson has succeeded in bringing the power of modern computers to the direct simulation of reacting systems. His extension of an earlier method for rarefied gas dynamics by Graeme Bird eliminates the use of differential equations and treats reaction kinetics on a probabilistic basis collision-by-collision. It is the method of choice for many low-density systems with coupled relaxation and reaction, and with non-equilibrium distributions. It has been applied to the complete simulation of detonations as well as to the prediction of ultra-fast detonations.
