Roothaan was born in
Nijmegen. He enrolled
TU Delft in 1935 to study
electrical engineering. Roothaan was studying physics at university when World War II broke out. As a Catholic citizen, he was temporarily allowed to continue studying. During World War II his brother Vic was involved with the Dutch Resistance, and when German soldiers came to the house looking for him they didn’t find Vic, but took Clemens Roothaan and his younger brother Jan instead. Later he and his brother Jan were sent to the
Vught concentration camp for suspected involvement with the Dutch Resistance. On September 5, 1944, the remaining prisoners of the camp (including the Roothaan brothers) were moved to the
Sachsenhausen camp in Germany ahead of the advancing Allies. Near the end of the war, the Sachsenhausen inmates were sent on a
death march which Roothaan's brother did not survive. While a prisoner of war he was able to pursue his studies in physics together with other professors and students under the formal guidance of
Philips. The work he was assigned to while cooperating with Philips was a foundation for his master's thesis. He obtained his master's degree in physics from
TU Delft on October 14, 1945. After that he moved to USA, where he did his PhD thesis with
Robert S. Mulliken from the
University of Chicago, on
semiempirical MO theory, while holding a post at
The Catholic University of America in Washington, D.C.. He realised that the then current approach to
molecular orbital theory was incorrect and changed his topic to what resulted in the development of the
Roothaan equations. Prof. Mulliken mentions this work in his Nobel lecture as follows: {{cquote|I tried to induce Roothaan to do his Ph.D. thesis on Hückel-type calculations on substituted benzenes. But after carrying out some very good calculations on these he revolted against the Hückel method, threw his excellent calculations out the window, and for his thesis developed entirely independently his now well known all-electron LCAO SCF self-consistent-field method for the calculation of atomic and molecular wave functions, now appropriately referred to, I believe, as the Hartree-Fock-Roothaan method. He had moved to the University of Chicago in 1949 and his PhD was awarded in 1950. He then joined the Physics Department of the University of Chicago. From 1962 to 1968 he was Director of the University of Chicago Computation Center. Later he was Professor of Physics and Chemistry at the University of Chicago. Since his retirement, in 1988, he had worked for the
Hewlett-Packard Laboratories in
Palo Alto, California, where his primary contribution had been in the development of the mathematical coprocessor routines for the
Itanium chip. His method of analyzing pipeline architecture had been unique and innovative and greatly admired in supercomputer circles around the world. In 1982 Roothaan became a correspondent of the
Royal Netherlands Academy of Arts and Sciences. He was a member of the
International Academy of Quantum Molecular Science and the Society of Catholic Scientists. He
turned 100 in August 2018 and died in June 2019. == Works ==