The Nobel Prize in Chemistry 1998 was divided equally between Walter Kohn "for his development of the density-functional theory" and John A. Pople "for his development of computational methods in quantum chemistry".
 

Density functional theory (DFT) is a quantum mechanical theory used in physics and chemistry to investigate the electronic structure of many-body systems, in particular atoms, molecules, and the condensed phases. Kohn played the leading role in the development of density functional theory, which made it possible to incorporate quantum mechanical effects in the electronic density. This computational simplification led to many insights and became an essential tool for electronic materials, atomic and molecular structure. W.Kohn's theoretical work has formed the basis for simplifying the mathematics in descriptions of the bonding of atoms, a prerequisite for many of today's calculations. John Pople is rewarded for developing computational methods making possible the theoretical study of molecules, their properties and how they act together in chemical reactions. He realised that if theoretical methods were to gain any significance within chemistry it was necessary to know how accurate the results are in any given case.Through significant improvements in the theoretical methodology at the end of the 1960s Pople designed a computer program which at a number of decisive points was superior to others' efforts. The prerequisites mentioned above could now be fulfilled and GAUSSIAN-70, as the program was called, very soon became widely used. Pople continued during the 1970s and 1980s to refine the methodology, at the same time building up a well-documented model chemistry. J. Pople developed the entire quantum-chemical methodology now used in various branches of chemistry.