He was previously the Hepburn Professor of Physical Science in the Center for Molecular Modeling at the University of Pennsylvania.

His education and career then moved to the United States in 1967 where he served as a Research Associate of Physics at Rutgers University in New Jersey.

Klein's research in computational chemistry, particularly statistical mechanics, intermolecular interactions, and modeling of condensed phases and biophysical systems, is among the most highly cited in the field.

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.

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.