Clemens Charles John Roothaan was born on 29 August, 1918 and passed away on 17 June, 2019 just short of his 101st birthday. He was a pioneer in the development and application of rigorous theory to the electronic structure of atoms and molecules. His theoretical approaches have also been extended to condensed phase systems. As well as establishing the theoretical foundations, he contributed a great deal to the development of numerical methods that made it possible to compute wavefunctions using digital computers. He guided the first calculations on atoms and molecules on digital computers using programs that he and his students and post-docs wrote.
Clemens received his PhD in 1950 at the University of Chicago under the sponsorship of Robert S. Mulliken. His thesis on the application of Molecular Orbital theory to the determination of wavefunctions for closed shell molecules was published in the Reviews of Modern Physics in 1951. His extension of the theory to open shell molecules was published in the same journal in 1960. One measure of the impact of these pioneering papers is that the first paper has been cited over 6800 times and the second over 2200 times; furthermore, they continue to be regularly cited, including several citations in 2019. In addition, Clemens was the author or co-author of many of the first papers describing mathematical methods for the calculation of integrals needed to determine molecular wavefunctions.
Shortly after his thesis was published, a conference or workshop was held at Shelter Island in September, 1951 where directions for progress in the theoretical discipline that what we now call Quantum Chemistry were formulated. A rather young Clemens attended this workshop together with many of the other pioneers in electronic structure theory and he was key to the discussions that lead to the very positive conclusions that were reached about the potential for Quantum Chemistry.
At the University of Chicago, the Laboratory of Molecular Structure and Spectra, LMSS, directed by Robert Mulliken and Clemens Roothaan was, for decades, a major center for theoretical research in the energies and properties of atoms and molecules. The students and post-docs who were members of LMSS were a productive group who, in turn, trained a new generation of physicists and chemists in electronic structure theory.
In a very real sense, CASCaM continues to build on the foundations that Clemens Roothaan laid down in the early years of Theoretical Chemistry at LMSS. Indeed, on the staff are two people who have directly collaborated with Clemens. We are all grateful to the contributions that Clemens made and we shall strongly miss him. He was a true giant in our discipline.
Please read the article "Prof. Emeritus Clemens C.J. Roothaan turns 100 on August 29th", written by Dr. David Bevington, currently posted on the University of Chicago website.