Robert G. Parr, emeritus professor at the University of North Carolina at Chapel Hill, passed away at the age of 95 on March 27, 2017. Parr was one of the founders of quantum chemistry, and his seminal contributions to the field included ab initio methods, semiempirical techniques, molecular spectroscopy, and density functional theory. Bob's influence perfuses quantum chemistry, including both semiempirical (a strategy he pioneered) and ab initio (a term he coined) approaches. Indeed, his work is so pervasive that it is easier to characterize his research style—mathematical elegance; deep intuition; minimalist writing; curiosity; prescience—than his research discoveries. He approached chemistry with the humility of one who recognizes that the discipline is so big that it can not be completely mastered, even by its most accomplished practitioners, and with the grace of one who regards advancing science as an intrinsically ennobling pursuit. Bob's scientific journey started as a molecular spectroscopist—guided by Paul Cross at Brown University (BS) and Bryce Crawford at the University of Minnesota (PhD)—working to compute molecular vibrational frequencies. This led to pioneering work on configuration interaction calculations in the early 1950s. He continued to work on ab initio methods during this decade (including seminal work on strong orthogonality and electron group functions), but this is also when he laid the foundations of semiempirical quantum chemistry. He established the zero-differential overlap approximation and showed how to rigorously derive π-electron effective Hamiltonians for conjugated hydrocarbons. The need to parameterize the integrals in semiempirical methods led to the Pariser–Parr–Pople model (1953), which was the first method to successfully predict the electronic spectra of sizable organic molecules. The Pariser–Parr–Pople Hamiltonian is still used today, most often in the simplified form that Hubbard introduced almost a decade later. Topics related to molecular spectroscopy continued to dominate Bob's work in the 1960s and early 1970s. During this period, he developed generalized Hellmann–Feynman theorems and the Simons–Parr–Finlan coordinate transformation. Nonetheless, more traditional quantum chemistry research (including the corresponding orbital transformation, the analysis of basis-set convergence, and the study of wavefunction asymptotics) continued apace. In the late 1970s, his interests shifted to (reduced) density matrices and density functional theory (DFT). These topics, alongside periodic forays into information theory, spectroscopy, wavefunction methods, and other topics, would dominate the last four decades of his life. More than anyone else, he brought DFT to fruition as a method for chemists for quantitative prediction and qualitative understanding of chemistry. He made key contributions to the mathematical framework of DFT, including the elucidation of the derivative discontinuity, the characterization of the exchange-correlation potential, and the development of functional-analytic techniques. He created computational methods, including the pervasive Lee–Yang–Parr correlation energy functional. He pioneered the idea that chemical concepts could, and should, be given precise mathematical definitions, and with Ralph Pearson he used the DFT framework to define chemical hardness and to clarify the hard/soft acid/base principle. He also used DFT to provide modern refinements to the concepts of electronegativity and frontier-molecular-orbital theory. Bob was beyond generous, both scientifically and personally. He would not agree to be a co-author unless he felt he had made an essential and specific intellectual contribution to a paper. Nearly half the papers describing research performed in his laboratory did not include his name, including classic papers on the Hartree–Fock molecular orbitals, the contracted Schrödinger equation, local thermodynamics, classical DFT, and electronegativity-equalization methods. Over the course of his professional career, he was Professor of Chemistry at Carnegie Institute of Technology (now Carnegie Mellon University; from 1948), Johns Hopkins University (from 1962), and the University of North Carolina at Chapel Hill (from 1974). He won numerous fellowships and awards, and was a member of the US National Academy of Sciences. Bob pursued science in an idyllic and romantic way, and it comes as no surprise that he also wrote poems. The late Vedene Smith liked to refer to the extended circle of Parr's students, postdoctoral fellows, senior visitors, and their associates as the Parr-tisans. He was not the only person to note that people who came through Bob's group had a certain je ne sais quoi. It was impossible to work with Bob and not be inspired by his irrepressible curiosity, his eagerness to try new mathematical tools and new intuitive strategies, his obsession with all things that were interesting and fundamental, and, most of all, his example as a scientific global citizen.
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Oct 1, 2023
Weimin Sun 
Open Access