Klein has advanced the field of computer simulation and modelling of molecular systems over a broad front. His early works focused on developing pragmatic intermolecular force fields to be used in computer simulation Monte Carlo and molecular dynamics (MD) simulations of molecular systems, such as water and aqueous solutions. During the 1980s and early 1990s, his group developed and elaborated algorithms and methodologies to enable the efficient and rigorous computer simulation of macromolecular systems. These seminal works have been influential and are very highly cited because of their broad utility. Every modern MD simulation code employs these algorithms in one form or other. Thus, modern molecular simulation studies of chemical systems ranging from surfactants to proteins and from lipid membranes to energy materials - including solid electrolyte fuel cells, and so-called “green” ionic liquids - take advantage of these algorithms. His pioneering simulation studies of surfactants, lipid membranes, and membrane-bound ion channels are noteworthy.