2017 Benjamin Franklin Medal in Physics presented to Marvin L. Cohen, Ph.D

Quantum mechanics is arguably the most important physical theory to have emerged in the 20th century. It, in principle, explains nature on the atomic level completely, specifically the interactions between electrons and the energy levels of atoms, necessary to understand the properties of atoms, molecules, and solids, all the way to living things. The fundamental laws were established already before 1930, but applying them to real systems, observable in the real world, has taken much longer. This was due, at least in part, to the computational complexity of these laws, which can only be solved by making careful approximations. Once one has these results, one has to extract the physical important consequences. Marvin Cohen has played a central role in making it possible today to use these equations in practical terms to explain and predict macroscopic properties of materials. His pioneering work started already in the 1960s, when he used the concept of "empirical pseudopotentials" to make accurate descriptions of the optical properties of a large group of semiconductors, based on existing measurements (= "empirical"). In the following years, the understanding of the interaction of electrons deepened, and he was in the 1970s and 1980s able to develop new theories and carefully made approximations that made the calculations independent of experimental data; he could truly start to predict the properties of materials by only knowing the identity of the atoms involved ("first principles"). The impact of this is hard to overestimate: It is now possible to predict the properties of novel materials before they are synthesized in the laboratory, making progress in the development of new functional materials much faster. Cohen has for 50 years been at the very forefront of this development, extending it to ever more complex materials. He has mentored a large number of young scientists, who, together with him, have continued to carry the field forward. Indeed, it is likely that Cohen and his intellectual descendants compose the most cited group of physicists in the world. Today, quantum mechanical calculations of solids are a large and ever increasing field, in size as well as importance. Cohen stands alone, through his contributions to the development of fundamental theory, to calculation schemes all the way to the application to real materials.