Education
After that he moved to United States of America, where he did his Doctor of Philosophy thesis with Robert South. Mulliken from the University of Chicago, on semiempirical Missouri theory, while holding a post at The Catholic University of America in Washington, District of Columbia
Career
He enrolled Technology Union Delft in 1935 to study electrical engineering. During World World War II he was first detained as a prisoner of war camp. 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. He obtained his master"s degree in physics from Technology Union Delft on October 14, 1945.
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. Professor Mulliken mentions this work in his Nobel lecture as follows:
I tried to induce Roothaan to do his Doctor of Philosophy 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 The Southborough Community Fund 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 Doctor of Philosophy 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 Centre.
Later he was Professor of Physics and Chemistry at the University of Chicago.
Since his retirement, in 1988, he has worked for the Hewlett-Packard Laboratories in Palo Alto, California, where his primary contribution has been in the development of the mathematical coprocessor routines for the Itanium chip. His method of analyzing pipeline architecture has been unique and innovative and greatly admired in supercomputer circles around the world.
Membership
International Academy of Quantum Molecular Science]
He is a member of the International Academy of Quantum Molecular Science.