POPLE, JOHN ANTHONY

(b. Burnham-on-Sea, United Kingdom, 31 October 1925, d. Wilmette, Illinois, 15 March 2004), chemistry, computational quantum chemistry.

Pople revolutionized the way that chemistry is practiced by making it possible to perform chemistry in the computer as a complement to the conventional chemistry of the laboratory. He was a giant in his chosen field of computational quantum chemistry, for which he was awarded the Nobel Prize in Chemistry in 1998. Pople was the person most responsible for making computational quantum chemistry usable by the community of chemists at large, and he dominated the scene in this area for five decades. By the time of his death in 2004, it would have been difficult to imagine any significant chemistry establishment in the world that did not make use of one or other of the Pople procedures.

Family Background and Early Education John Pople was born in Burnham-on-Sea in southwest England in 1925. His paternal great-grandfather had been a successful businessman, and one of these businesses, a clothing shop, was inherited by John’s grandfather and subsequently by his father. John’s mother came from a farming background and, although most of her relatives remained in farming, she became a tutor and a librarian. John’s parents were ambitious for their children and sent John and his brother to Bristol Grammar School, some thirty miles away. This provided a challenging experience during the war years, with the occurrence of heavy air raids in the vicinity of Bristol, but it was an excellent school that led to an excellent education.

John’s intense interest and talents in mathematics were apparent at an early age. However, he describes in his Nobel autobiography how he introduced deliberate errors into his mathematics exercises at Bristol Grammar School so as not to appear too smart. It was not until a new mathematics teacher arrived on the scene and created a particularly challenging test that John succumbed to temptation and turned in a perfect paper, including multiple solutions to several of the problems. Despite the remarkable achievements that were to follow, Pople always retained his modest manner.

Once his talents were recognized, Pople was encouraged to apply for a scholarship to study at Cambridge University. Ironically, during his last two years at Bristol Grammar School, he abandoned chemistry to concentrate on mathematics and physics. Many years later, in the 1960s, this was to lead to an amusing situation where his initial application to join the American Chemical Society was rejected because he had not completed the requisite number of courses! Pople entered Cambridge in 1943 and completed his undergraduate education in May 1945.

Cambridge and the National Physics Laboratory Pople began life as a research student in theoretical chemistry at Cambridge University with Sir John Lennard-Jones in July 1948. His interest in pure mathematics had begun to wane by that time, so he decided to apply his mathematical skills to chemistry. He worked on the structure of liquid water and was awarded a PhD in 1952. During this early period in Cambridge, Pople was also learning to play the piano. His teacher was Joy Bowers, and they were subsequently married in 1952, a partnership that would continue for almost fifty years. Pople was appointed a Research Fellow at Trinity College in Cambridge in 1951 and a lecturer on the mathematics faculty in 1954. The germs of the ideas that were to provide the focus for the central theme of his life’s work, namely, developing mathematical models that would be capable of describing all of chemistry, were sown during this period. However, there were two significant digressions along the way.

In 1955 Pople developed an interest in the then-emerging technique of nuclear magnetic resonance (NMR) spectroscopy, a forerunner to the magnetic resonance imaging of the early twenty-first century. He spent two summers at the National Research Council of Canada working on the theoretical basis of NMR. This work led to seminal contributions, including a landmark monograph on the subject, and constituted Pople’s main research activity during his final years in Cambridge.

The second digression arose because Pople had become dissatisfied with his mathematics teaching post and therefore sought a position with greater scientific content. He eventually accepted an appointment as head of the Basic Physics Division of the National Physical Laboratory in Teddington, near London. However, this also proved unsatisfactory, as he found the administrative load interfered too much with his research activities. Pople described the time in the National Physical Laboratory as a fallow period in his scientific life.

Carnegie Mellon University and the Brain Drain In 1961 Pople was invited by Robert Parr, himself a leading international quantum chemist, to be the Ford Visiting Professor at the Carnegie Institute of Technology in Pittsburgh for the 1961–1962 academic year. This appointment was a thoroughly enjoyable experience for Pople and his family; it also made him aware of the opportunities for scientific advancement in the American environment. When Parr decided to leave for Johns Hopkins University in 1963, his position at Carnegie Tech was offered to Pople, who accepted it. In soliciting Parr’s advice on this matter, Pople had written: “Like most of the academic community, I am looking for a lively scientific environment, a good salary, a tolerable place to live in, and freedom from administrative chores.” This is a realistic summary of Pople’s priorities (particularly his disdain for administration) and his relatively undemanding requirements throughout his academic life. The decision in favor of Carnegie Tech ahead of competing offers from Chicago and Princeton was probably decided by his enjoyable earlier experience in Pittsburgh. The departure from the United Kingdom of someone of Pople’s stature was decried in the U.K. press with the headline “Another Brain down the Drain,” but it was welcomed in the Pittsburgh Press with the headline “‘Drain’ Flows Our Way: Top British Brain Coming to Tech.” Leaving the United Kingdom was a painful decision for the Poples, and they attempted to compensate in some respects with annual summer visits “back home.” The appointment in 1964 marked the beginning of a thirty-year relationship (1964–1993) with Carnegie Tech (subsequently to become Carnegie Mellon University after a merger with Mellon Institute). Virtually all of Pople’s prize-winning work was carried out while he was a professor at Carnegie Mellon from 1964 to 1993.

By 1981 all the Pople children had left home, and John and Joy set up house in Chicago, to be close to their daughter Hilary. John initially commuted from Pittsburgh, while also holding an adjunct appointment at Northwestern University. When he retired from Carnegie Mellon in 1993, he completed the move to Chicago, taking up a full faculty position at Northwestern University. Pople also had close connections with the Australian National University in Canberra, which he described as his “second academic home—a great place for relaxed contemplation,” and which he visited on nine occasions during the 1980s and 1990s. He also had close ties with Israel and Germany.

Pople’s Main Research Pople’s long-term goal was the creation of theoretical models for studying chemistry. It was known a long time previously, and enunciated by Paul Dirac in 1929 with an often-quoted statement, that the laws of quantum mechanics could in principle be used to predict all of chemistry: “The fundamental laws necessary for the mathematical treatment of a large part of physics and the whole of chemistry are thus completely known, and the difficulty lies only in the fact that application of these laws leads to equations that are too complex to be solved” (“P. A. M. Dirac,” 1929, p. 714). What Pople did was to convert the principle to a practical reality. His aim was to enable chemists at large to be able to straightforwardly predict the properties of molecules, such as molecular structures and the way that molecules reacted with one another, by using a computer rather than by carrying out experiments.

Pople went about this task by first formulating the essential characteristics of an acceptable theoretical model. For example, such a model should be unique, well-defined, unbiased, objective, and widely applicable, and it should also satisfy some more technical requirements (e.g., be “size-consistent”). If the model performs satisfactorily in systematic comparisons with available experimental data, it may then be used with confidence to make predictions in cases where experimental data are not available.

Pople then proceeded to design a series of theoretical procedures that could be used as the basis for the model. His first attempt in this direction in 1953 paralleled studies by Rudolph Pariser and Robert Parr in the United States, and was known collectively as “PPP theory.” This was not a complete model because it could only be satisfactorily applied to conjugated organic molecules. However, in the precomputer age, that was the limit of what was feasible.

Pople resumed his work on implementing model chemistries when he moved to Pittsburgh in 1964. His next step was the introduction of a series of methods that were termed semiempirical. These simplified the quantum chemistry calculations by introducing approximations, while attempting to compensate for the approximations by including empirical parameters based on experimental data. Examples of these procedures include the complete neglect of differential overlap (CNDO) and intermediate neglect of differential overlap (INDO) methods. They represented genuine chemical models in that they could treat a wide variety of molecules and provide predictions of properties such as molecular structures and energies. However, they were of limited predictive value because of uncertainties regarding the consequences of the approximations made in their formulation.

By this time, the explosive growth of high-speed computers had begun, and Pople recognized that the use of computers would be the basis for the future of quantum chemistry calculations. He also recognized that the theoretical developments needed to move hand in hand with efficient computer code, and he mastered this new skill quickly and with enthusiasm. The initial university computer purchased after Pople joined Carnegie Mellon University was a Control Data Corporation (CDC) 1604 in 1966. It is of interest to note that the CDC 1604 had a delivered speed of 0.03 MFlops and a memory of 0.2 Mbytes; it was approximately 100,000 times slower and had 5,000 times less memory than an average laptop computer in 2006—making Pople’s achievements in those early days all the more remarkable. Subsequently, a Univac 1108 was purchased in 1971, and in 1978 Pople was able to acquire the first VAX/780 minicomputer from Digital Equipment Corporation.

Pople felt that it would be desirable to implement molecular orbital theory procedures that did not require experimental data in their formulation, and he set about moving from semiempirical to ab initio methods. The term ab initio literally means “from the beginning” and in this context refers to the fact that the methods are based solely on the laws of quantum mechanics (in particular the Schrödinger equation) without recourse to any experimental information other than the values of fundamental constants such as the speed of light and the charge on the electron. Ab initio procedures had been in existence for many years, but the available general programs were very slow and thus severely limited the range of chemical systems to which they could be applied. In comparison to these ab initio programs, Pople’s CNDO procedure was faster by a factor of approximately 1,000.

Pople’s entry into the field and his contributions in the late 1960s and early 1970s changed the face of ab initio quantum chemistry. He designed highly efficient computer codes, involving features such as much faster routines for evaluation of the integrals that are required for the calculations. He created efficient purpose-built basis sets, a necessary input into the ab initio molecular orbital theory calculations. When the code (named Gaussian 70) was first completed in 1970, it was approximately 100 times faster than the most popular of the existing programs. Pople’s development and release of the Gaussian 70 program marked a turning point in the field. Broad theoretical studies could now be carried out on real chemical problems on a scale that had not been previously possible.

The developments by Pople from 1968 to 1972 laid the foundations for the widespread use of ab initio calculations by the chemical community, as Gaussian 70 rapidly gained acceptance in chemical laboratories around the globe. Some of the reasons for this immediate popularity were that Gaussian 70 was fast and easy to use; the performance of the internal basis sets, with names such as STO-3G, 4-31G, and 6-31G* had been thoroughly tested and could therefore be employed where appropriate with confidence; a wide selection of fundamental examples of the types of problems that could be tackled and some of the important strategies that might be used in tackling them had been published; and through John Pople’s generosity, the program was made available at minimal cost for general use via the Quantum Chemistry Program Exchange. These foundations were strengthened and expanded in subsequent years.

The initial molecular orbital procedures that had been coded into Gaussian involved the Hartree-Fock method. This method does not allow for so-called correlation in the motion of electrons. In the late 1970s, Pople formulated an efficient method for including such electron correlation in the calculations, based on the perturbation theory of Møller-Plesset, dating back to 1934. Electron correlation was certainly not a new concept, but it had previously been the province of specialized theoreticians. Pople had found a way to incorporate it straightforwardly and at low cost, thus making it readily usable by the nonspecialist. Simplicity and efficiency were two of the ever-present hallmarks of the Pople approach.

Even in the more recently popularized density functional theory, in which he was formally only a minor player (the dominant contributor being Walter Kohn, with whom Pople shared the 1998 Nobel Prize in Chemistry), Pople systematized the way in which such calculations were carried out and incorporated them into the Gaussian program. This undoubtedly helped significantly in accelerating the widespread acceptance and use of density functional theory in chemistry.

Pople made continuing innovative contributions to the development and application of ab initio quantum chemistry throughout his research career. In the final decade, he focused on formulating model theories that could provide thermochemical information of high accuracy at modest computational expense. These models were named Gaussian n (Gn) and had reached G3 at the time of his death. They were able to predict thermochemistry with an accuracy that rivaled that of experimental work.

Computational quantum chemistry has become used by myriad chemists across a variety of fields. It has become a viable adjunct to experiment in the pursuit of chemistry, and it is used to help solve problems of a fundamental nature. It is also being used increasingly by industrial companies in more practical situations, such as in the design of new drugs and of new materials. The computational approach to chemistry allows the study of substances that might be difficult to examine experimentally, for example, because they have a very short lifetime or because they are dangerously toxic or explosive. The computer is oblivious to such hazards, and the increasing applicability of the computational approach to chemistry has been helped by massive and continuing increases in computer power. The individual most responsible for bringing about this change in the way chemistry is practiced was John Pople.

Apart from being a brilliant researcher, Pople was also a great communicator, as anyone who ever heard him lecture would attest. Some people have the knack of making simple things complicated; Pople had the ability to keep simple material simple and the gift of making complicated material appear simple as well.

Pople’s achievements received widespread recognition. He was elected a Fellow of the Royal Society in 1961, and was a founding member of the International Academy of Quantum Molecular Science in 1964, serving as president from 1977 to 2000. He was elected to the U.S. National Academy of Sciences in 1977. He received the Schrödinger Medal of the World Association of Theoretical and Computational Chemists in 1987. In 1992 he received Israel’s premier prize, the Wolf Prize, and in 1993 he was elected a Corresponding Member of the Australian Academy of Science. He shared the 1998 Nobel Prize in Chemistry with Walter Kohn in 1998, and was awarded the Copley Medal of the Royal Society in 2002. In 2003 he was knighted by Queen Elizabeth II, becoming Knight Commander of the Order of the British Empire.

BIBLIOGRAPHY

WORKS BY POPLE

With William G. Schneider and Harold J. Bernstein. High Resolution Nuclear Magnetic Resonance Spectroscopy. New York: McGraw-Hill, 1959. A significant early monograph on NMR.

With David L. Beveridge. Approximate Molecular Orbital Theory. New York: McGraw-Hill, 1970. The first major book on semiempirical theory.

With Warren J. Hehre, Leo Radom, and Paul von R. Schleyer. Ab Initio Molecular Orbital Theory. New York: John Wiley, 1986. The classic text in the field.

“John Pople: Autobiography.” In Les Prix Nobel, 187–193. Royal Swedish Academy of Sciences, 1998. Pople’s life story, written with his renowned clarity. A major source for this article.

“Quantum Chemical Models.” Angewandte Chemie, International Edition 38 (1999): 1894–1902. Pople’s Nobel lecture.

OTHER SOURCES

Buckingham, A. David. “John Anthony Pople, KBE.” Biographical Memoirs of Fellows of the Royal Society 52 (December 2006). A detailed biographical memoir written by Pople’s first PhD student and longtime friend.

Frenking, Gernot, Paul von R. Schleyer, Norman L. Allinger, et al. “Publishers Note: Sir John A. Pople.” Journal of Computational Chemistry 9 (2004): v–viii. Personal reflections on Pople.

Mangravite, Andrew. “The Flight of the Boffin.” Chemical Heritage (Summer 2004): 27–29. This article gives an interesting account of Pople’s move to the United States.

“P. A. M. Dirac.” Proceedings of the Royal Society (London) 123 (1929): 714.

Radom, Leo. “John A. Pople: The Early Days.” Journal of Physical Chemistry 94 (1990): 5439–5444. A detailed account of the critical developments leading up to the release of Gaussian 70.

———. “John A. Pople (1925–2004).” Nature 428 (2004): 816. An obituary that was a major source for the present article.

Leo Radom