Henry Frederick "Fritz" Schaefer III (born June 8, 1944) is an American computational, physical, and theoretical chemist.

[13] Schaefer was educated at the Massachusetts Institute of Technology (MIT), where he initially intended to specialize in chemical engineering.

At Stanford, he worked with chemist Frank E. Harris on ab initio electronic structure theory and quantum chemistry.

Through collaborations with other researchers, he also gained access to resources at the University Computing Company (UCC) in Palo Alto, which had a UNIVAC 1108.

During his time at Berkeley, Schaefer published 375 papers and several books, including The Electronic Structure of Atoms and Molecules: A Survey of Rigorous Quantum Mechanical Results (1972) and Quantum Chemistry: The Development of Ab Initio Methods in Molecular Electronic Structure Theory (1984), a survey of research with commentary.

With the help of an IBM 3090-200E mainframe (as well as later models) he and his research group developed various computer-based methods for advanced quantum chemistry.

[26] Research within the Schaefer group involves the use of computational hardware and theoretical methods to solve problems in molecular quantum mechanics.

His contributions to the field of quantum chemistry include a paper challenging, on theoretical grounds, the geometry of triplet methylene as assigned by Nobel Prize-winning experimentalist Gerhard Herzberg; the development of the Z-vector method simplifying certain calculations of correlated systems; and a wide body of work undertaken in his research group on the geometries, properties, and reactions of chemical systems using highly accurate ab initio quantum chemical techniques.

In 1992, he was awarded the Centenary Prize of the Royal Society of Chemistry, London, for being "the first theoretical chemist successfully to challenge the accepted conclusion of a distinguished experimental group for a polyatomic molecule, namely methylene.

[34] On March 18, 2014, Schaefer received the American Chemical Society Peter Debye Award in Physical Chemistry.

He returned to India to give his CRSI Honorary Fellow award lecture on February 6, 2016, at Panjab University in Chandigarh.

[37] Schaefer wrote the forward to William A. Dembski's 1998 book Mere Creation: Science, Faith and Intelligent Design.