text: Klein was a researcher at the National Research Council of Canada from 1968-1987 and joined the faculty of the University of Pennsylvania in 1987. 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. He received the Aneesur Rahman prize in 1999, which is the highest honor given by the American Physical Society
  for work in computational physics, and was elected to the United States National Academy of Sciences in 2009.