Clemens C. J. Roothaan (August 29, 1918 – June 17, 2019) was a Dutch physicist and chemist known for his development of the self-consistent field theory of molecular structure.
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[3] as follows: I tried to induce Roothaan to do his Ph.D. thesis on Hückel-type calculations on substituted benzenes.
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.