Working with DuPont chemist Rudolph Pariser, Parr developed a method of computing approximate molecular orbitals for pi electron systems, published in 1953. Since an identical procedure was derived by John A. Pople the same year, it is generally referred to as the Pariser–Parr–Pople method or PPP method. The PPP method differed from existing structural chemistry thinking (which advocated maximum overlap principle) by advancing the concept of zero differential overlap approximation.
By 1978 Parr had realized that density functional theory (DFT) would be extremely useful in quantitative calculations of chemical and biological systems, especially those with high molecular weights. In 1988 Parr, Weitao Yang and Chengteh Lee produced an improved DFT method which could approximate the correlation energy of systems. The LYP (for Lee–Yang–Parr) functional theory is now one of the most cited papers in the chemical literature.
In 1963 Parr published Quantum Theory of Molecular Electronic Structure, one of the first books to apply quantum theory to chemical systems.
In 1989 he and Yang published Density Functional Theory of Atoms and Molecules, now considered the basic textbook on DFT.
