Clemens C. J. Roothaan

Clemens C.J. Roothaan (born in August 29, 1918 in Nijmegen) is a Dutch physicist.

He enrolled TU Delft in 1935 to study electrical engineering. During World War II he was first detained in a prisoner of war camp. Later he and his brother were sent to the Vught concentration camp for 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:

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.

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