Donald Hubert Secrest

Donald Hubert Secrest

Donald Hubert Secrest, Professor Emeritus of Chemistry at the University of Illinois at Urbana-Champaign, died unexpectedly of pneumonia on 10 May 2014 in Philadelphia after long suffering from Lewy Body Dementia. Don, a pioneer in computational chemical physics, was well known for his fundamental advancement of practical methods by which accurate quantum mechanical calculations of molecular collisions could be routinely performed.

Don was born on 3 January 1932 in Akron, Ohio. He attended the University of Akron and obtained his bachelors degree in chemistry in 1955. Subsequently, he spent a year studying in Göttingen, Germany on a Fulbright Fellowship, before pursuing graduate study in the Theoretical Chemistry Institute at the University of Wisconsin–Madison. He earned his PhD under Joseph Hirschfelder in 1961. He then moved to the University of Illinois, where he taught for 39 years, supervising over 20 PhD dissertations.

Don’s research in the 1960s was a model of both vision and precision. With his student B. Robert (Bob) Johnson in 1966 he obtained the first exact quantum mechanical solutions for the vibrational excitation of a diatomic molecule in a collinear collision with an atom. This work established, for the first time, exact quantum mechanical benchmarks from which it was discovered that some of the approximations then in widespread use actually converged to the wrong answers in limits under which they were thought to be accurate. Shortly thereafter, Johnson and Secrest performed the first exact calculations in three dimensions for rotational excitation in atom-diatom collisions, paving the way to routine first-principles calculations of energy transfer in molecular collisions.

Don’s theoretical results had a profound impact on the molecular beam experiments then underway in Göttingen. It opened up the field and led to a great outburst of activity in the 1970s to find accurate and efficient approximations for computing inelastic and reactive molecular scattering. As the 1970s dawned, the advent of faster computers considerably expanded the realm of collision problems that were accessible to accurate quantum mechanical solution. It was clear that a major impediment to the calculation of most problems was the computational expense in solving the coupled differential or integral equations, which increased as the cube of the number of states included. Through physical insight into the scattering process several very effective approximations were devised to substantially reduce the number of equations that needed to be solved.

Motivated by the fact that the proliferation of states was principally due to the closely spaced molecular rotational states, Pack and coworkers, in 1973, proposed the infinite-order sudden approximation, which eliminated the rotational coupling and thereby enabled the solution of uncoupled equations. Kouri and coworkers, in 1974, proposed the coupled states approximation, in which the orbital angular momenta alone are decoupled, retaining the molecular rotational state coupling. In a groundbreaking paper in 1975, Don noted that these seemingly distinct approaches are actually particular realizations of a broader class of angular momentum decoupling approximations. This observation stimulated much activity to understand in physical terms exactly how these approximations worked, and how they might be applied to problems for which the exact calculations were beyond computational reach. Don’s extraordinary insight subsequently made it possible for him to suggest major improvements of these approximations.

Throughout his entire career Don made fundamental contributions to molecular scattering theory. In later years he and his students developed fundamentally sound theoretical methods for attacking a variety of issues. Among them were very successful efforts to establish the exact quantum mechanical Hamiltonians for polyatomic molecules, which were needed in order to carry out the associated scattering calculations.

His papers were models of clarity and physical insight, the key ideas always derived without unnecessary mathematical complexity. One could always be sure that his results were rigorously correct and reliable.

In addition to being a creative scientist, Don was an exceedingly generous teacher. In working with his students and postdocs, Don often had flashes of insight, for which he would later give the credit to his younger coworkers. This practice of sharing credit seemed to take no special effort on Don’s part; rather, it was as if he truly could not remember who had thought of what.

Don was particularly devoted to his wife, Masako, who died long before him, and to his children, Hideko and David. He was a fine gentleman who enjoyed people and saw the best in everyone. We have lost an inspiring colleague and a wonderful teacher who will long be remembered.

Donald J. Kouri
University of Houston
Houston, Texas

Kelly D. McLenithan
Los Alamos National Laboratory
Los Alamos, New Mexico

Russell T Pack
Los Alamos National Laboratory
Los Alamos, New Mexico

J. Peter Toennies
Max Planck Institut für Dynamik und Selbstorganisation
Göttingen, Germany