On the Origins of the PPP Method
E.I. du Pont de Nemours & Co., Central Research & Development Department, Experimental Station,
Wilmington, Delaware 19898
Often have I been asked about the origins of what is now called “The PPP Method.“
Specifically, how and why would a chemist such as I was at the Jackson Laboratory
of E.I. du Pont de Nemours & Co. get together with Bob Parr, an associate professor
at Carnegie Tech, to work on a semiempirical quantum theory?
My interest began in
the spring of 1951, shortly after my employment at Du Pont‘s Jackson Laboratory.
As the research laboratory of Du Pont‘s Organic Chemicals Department (which no
longer exists under that name), the work at Jackson Laboratory covered the full range
from basic science to pilot plant production. It was the site of many of Du Pont‘s key
inventions and developments, such as the fluoropolymer resins and elastomers, and it
was also the laboratory where Charlie Pedersen, the 1987 Nobel laureate in
Chemistry, worked. Now in retrospect, Jackson Laboratory can certainly claim an unusual
record for creative achievement. lt was in this environment that I embarked on my
career as a recent graduate from the University of Minnesota.
At the time of my arrival at Jackson Laboratory, in December of 1950, a large part
of the effort was devoted to the syntheses and evaluations of new organic dyes for
Du Pont‘s recently developed synthetic fibers, which were beginning to achieve
substantial commercial significance.
My Ph.D. thesis background in photochemistry
(under Professor Robert L. Livingston at the University of Minnesota) appeared
wellsuited to this effort. As one of a very few physical chemists, 1 was assigned to characterize some of the dyes being synthesized. However, the capability of instrumentation
was quite limited at that time, and the number and variety of dyes being synthesized
by my many organic chemist coileagues seemed overwhelming. I soon developed a
strong desire to gain a better understanding of structure/property relationships
particularly with regard to the color of dyes — since 1 hoped that such understanding
might save my coileagues a considerable amount of work in searching for the desired
structures. Also, perhaps naively, I believed that quantum chemistry should contain
the answers to many of our problems.
My management (especialiy John Tinker, Laboratory Director, as well as his
assistant, H. E. Schroeder, and my supervisor, A. V. Willet) was very supportive of my
ideas and encouraged me to do what I could. Since my experience in the field was
limited to the few courses at the University of Minnesota as taught very inspiringly
by Bryce Crawford, I felt that I needed someone to consult and to help me in my new
endeavor. Thus, in July of 1951 I approached Bob Parr, wbo had done his thesis
research under Crawford on the most advanced quantum mechanical calculations at that
time on simple organic molecules, and whom I had learned to respect at Minnesota as
a friend and teacher. I remember our first meeting in his office at Carnegie Tech in
Pittsburgh, and in retrospect 1 have felt especially grateful to Bob that he encouraged
me in my youthful enthusiasm for calculating the spectra of complex dyes when the
state of the art at the time was hardly adequate to explain the spectrum of ethylene!
Bob and I strongly believed that electronic repulsions would need to be included
specifically in any theory which was going to be predictive in a quantitative way. But this was practically impossible for any molecule larger than benzene, especially with
the limited computers of die early 1950s.
The breakthrough came when Bob Parr invented what was later termed the “Zero Differential Overlap (ZDO)“ approximation
. It was in November 1951 when I received Bob‘s letter about this, and in
subsequent conversations I recall his explaining to me how elegantly simple the application
I quickly went to work, and within some days we had calculated
benzene — and even napthalene with some difficulty — all on a mechanical desk calcula-
My excitement was unbounded. But soon I realized that although the results of
these calculations agreed very well with previous complete theoretical calculations,
they were still far from being in agreement with experiment.
In taking the next step,
my training as an experimentalist came in handy. What values should these integrals
have in order to fit experiment? With ZDO it was simple enough to adjust quantum
mechanical parameters to fit die expenmental data.
I was mentally well prepared for the parameters so obtained to have some rather
bizarre values. lt was indeed an exciting discovery that die values seemed not unreasonable, although quite different from the then accepted, theoreticaily calculated
values. In particular, the one-center pi-electron repulsion integral (11|11) assumed an
“experimental“ value of about 11 ev versus the “theoretical“ 17 ev. What a thrill it
was to discover that the value of 11 ev could be explained in terms of the valence
state disproportionation reaction:
2Co = C — + C+,
which yields the equation (11|11) = I - A or about 11 ev!
Bob visited me in Wilmington on December 13, 1951. Our discussion that day
lasted well into the night in his room at the Hotel Du Pont, where we were
convincing ourseives that (11|11) of about 11 ev was indeed totally reasonable. Wasn‘t it completely compatible with the “atoms in molecules“ concept which was being
advocated so convincingly by Bill Moffitt? As I wrote at that time, “the effect of the
sigma-electrons can be approximately taken into account without any additional complication to the mathematics of the ir electron approximation by changing the value of
primarily one Coulomb repulsion integral.“ We could also rationalize why some of
the other integrals should also be adjusted although to a much lesser degree .
The new empirical integrals together with the ZDO approximation still required,
however, the advent of the modern solid-state, high-speed computer to make our
method practically applicable to larger molecules. After many frustrating experiences with vacuum tube—based and card-programmed computers, I remember well the
excitement I felt when, luckily for us, IBM installed its first modern computer, the
“701,“ in New York City. I was fortunate to be among the first to learn to use this
computer and to develop a program for our method (which I had to write completely
in machine language).
Bob and I presented our work at the Symposium on Molecular Structure and
Spectroscopy, held at Ohio State University on June 9, 1952 [3,4]. Ours were the first
two papers on the first day of the symposium. I do not recall any questions from the
audience following our presentations. At first, this silence seemed disheartening.
Then, Professor Robert Mulliken stood up to say words to the effect that our papers
had presented a significant advance. He had made my day.
Application of our method to the spectra of the polyacenes soon followed, with
results which were enticingly close to experiment and also predicted the location and
assignments of a number of excited states. In addition, some general regularities for
alternate hydrocarbons were derived. These results were presented at the 1954 Ohio State Symposium on June 14 . At this and subsequent meetings, it became apparent that expenmentalists were paying attention to our work. I especially recall highly
informative discussions with Donald S. McClure, who was doing pioneering work in
assigning electronic transitions, particularly for naphthalene. Our calculations for the
lower singlet transitions were in spectacular agreement with his findings.
My last calculation was that for azulene, a nonalternate hydrocarbon, presented at
the Molecular Quantum Mechanics Conference in Austin, Texas, on December 8,
1955, and later pubiished . Again, McClure‘s and other experimental work
supported our results, and I concluded that “the present theoretical approach is applicable
to azulene, a polar molecule, with about the same accuracy as for naphthalene, a
nonpolar molecule. This is most encouraging.“
In retrospect this meeting in Texas, organized by F. A. Matsen, was indeed a
memorabie one for me. The pioneers of quantum chemistry were there, inciuding
C.A. Coulson, B.C. Crawford, J.0. Hirschfelder, G. E. Kimball, M. Kotani,
H. Kuhn, H.C. Longuett-Higgins, J.E. Mayer, K.S. Pitzer, J.R. Platt, B. Pull-
man, and J.C. Slater, as weil as my contemporaries, such as F.0. Eliison, I. Fischer-Hjaimars, H.H. Jaffe, M. Kasha, H.H. McConnell, W. Moffitt, C.C.J. Roothaan,
K. Rüdenberg, H. Shull, W. Simpson, and of course Bob Parr and John Popie.
This was also where I first discussed my work in some detail with Per-Olov Löwdin
of Uppsala, who was to become an outstanding contributor, leader, and organizer in
the field. lt was the last quantum mechanics meeting in which I actively participated.
My work with Bob Parr in developing our theory was nearing its conclusion. The
seven papers which we pubiished [1—7] presented the essence of our research. These
papers, together with publications by John Pople and coworkers , form the early
foundations of our method. We had started to show that the application of semiempirical quantum mechanical methods to the quantitative prediction of eiectronic structure
and spectra in molecules of interest to chemists was indeed a reaiistic objective.
already mentioned, in Jackson Laboratory during the development of our method, I
was interacting with a broad spectrum of organic chemists, including several consultants. Among the consultants was John D. Roberts, then a professor of organic
chemistry at the Massachusetts Institute of Technology, who was later to achieve world
renown in his field. Roberts took a keen interest in my work, and in fact learned to
perform calculations by our method. He, as weil as one of his postdoctorate students,
Andrew Streitwieser, who also consulted for us, became important pioneers for
applying semiempiricai moiecular orbital methods to chemistry.
At about this time, in the mid-1950s, research directions were changing in Jackson
Laboratory. Much more emphasis was being placed on polymer research, specifieally
on elastomers. I was being challenged by the many new problems in this field and
initiated some research on polymers. In 1954 I accepted the position of Research Supervisor of a newly formed group in elastomer physics and physical chemistry;
however, I continued to follow developments in quantum chemistry and to convey my
knowledge of this field to others in Du Pont. Among these was Howard E. Simmons,
a physical organic chemist, who had joined the Company in the corporate research
laboratory in 1954. He took a strong interest in our theoretical work and launched a
successful program to develop and apply our method to a variety of complex organic
molecules. Within Du Pont, the further development of our theory was in good
hands. Many years later, as Vice President and head of the Central Research and
Development Department, Howard became my boss.
Although Bob Parr and I did not quite meet our original goal of predicting the
color of complex organic dyes, it is very gratifying to know today that our method,
with its many refinements and extensions as developed subsequently by others, has
found such wide applicability .
The present volume gives me a deep sense of satisfaction and gratitude that the
seeds we may have planted more than 30 years ago continue to be of use to today‘s
scientists in such a variety of endeavors.
 R.G. Parr, “A method for estimating electron repulsion integrals over LCAO MO‘s in complex unsaturated molecules.“ J. Chem. Phys. 20, 1499 (1952). Received July 1, 1952.
 R. Pariser, “An improvement in the pi-electron approximation in LCAO MO theory.“ J. Chem. Phys. 21, 568 (1953). Received January 12, 1953.
 R. Pariser and R.G. Parr, “A semi-empirical theory of the electronic spectra and electronic structure of complex unsaturated molecules. 1.“ J. Chem. Phys. 21, 466 (1953). Received September 4, 1952.
 R. Pariser and R.G. Parr, “A semi-empirical theory of the electronic spectra and electronic structure of complex unsaturated molecules. II.“ J. Chem. Phys. 21, 767 (1953). Received December 11 1952.
 R. Pariser, “Theory of the electronic spectra and structure of the polyacenes and of alternate hydrocarbons.“ J. Chem. Phys. 24, 250 (1956).
 R. Pariser, “Electronic spectrum and structure of azulene.“ J. Chem. Phys. 25, 1112 (1956).
 R.G. Parr and R. Pariser, “On the electronic structure and electronic spectra of the ethylene-like molecules.“ J. Chem. Phys. 23, 711 (1955).
 J. A. Pople, Trans. Faraday Soc. 44, 1375 (1953) and subsequent publications.
 Our original papers were among the five most cited publications of the 1950s in chemistry and physics
during 1961—1977. The two basic publications, Refs. 3 and 4, were cited 2450 times in this period, as
reported in ISI, Current Contents, 1977, and were recognized as a Citation Classic. In 1978—1987
these two papers have been cited an additional 880 times, while Refs. 1—7, were cited 1200 times in
this latest 9-year period. (set in bold type by this website - U.A.)
Dr. Rudolph Pariser is currently engaged as a consultant in the areas of polymer and advanced materials sciences and in research management.
As of January 1989, he retired as Director for Advanced Materials Science in the
Central Research & Development Department of E.I. du Pont de Nemours & Co., Inc.
In this capacity he directed research in the following areas: electronic packaging;
information storage and imaging; optical communication; specialty polymers for
biomaterial, electronic, high-perfonnance structural and packaging applications;
composite materials; and high-temperature superconductors.
Dr. Pariser joined the Du Pont Company in 1950 as a research chemist. His early
work was primarily in the field of quantum chemistry and led to the development of a
theory now known as the “Pariser-Parr-Pople“ method, which is still widely quoted
and forms the basis for many of today‘s quantum chemical calculations.
Dr. Pariser‘s later research was in the polymer science area. In 1954, he was
named Research Supervisor and since then has held a number of polymer research
management positions. In 1974 as Director, Pioneering Research, Elastomer
Chemicals Department, he managed the development of several new products, including
“Hytrel“ polyester elastomers, “Vamac“ ethylene acrylic elastomer, “Kalrez“ perfluoroelastomer parts, and several new “Viton“ fluoroelastomers. In 1980, Dr. Pariser
became Research Director of the newly formed Polymer Products Department. In 1981,
he became Director for Polymer Science in the Central Research & Development
Born in Harbin, China, Dr. Pariser received a BS in chemistry from the University
of California, Berkeley in 1941 and a PhD in physical chemistry from the University
of Minnesota in 1950. He is married to the former Margaret Louise Marsh.
Dr. Pariser is on the Advisory Board, Journal of Polymer Science; Editorial Board,
New Polymeric Materials; IUPAC CHEMRAWN Committee; National Research
Council Committee on Materials Science & Engineering; and the National Science
Foundation Advisory Comrnittee on materials. He has served on the National
Research Council as Co-Chairman, Ad Hoc Panel on Polymer Science & Engineering
(1979—81); the Committee on Chemical Sciences (1979—82); and Co-Chairman,
Panel on High Performance Composites (1984). He has also served as Associate
Editor of the Journal of Chemical Physics, Chemical Physics Letters, and Du Pont
Innovation. His professional associations include the American Chemical Society, Society
of Rheology, American Association for the Advancement of Science, New York
Academy of Science, American Physical Society, Phi Lambda Upsilon, Sigma Xi,
National Research Council, and National Science Foundation.
 “The Chlorophyll-Sensitized Photo-Oxidation of Phenylhydrazine by Methyl Red. II Reactivity of the Several Fonns of Methyl Red.“ J. Am. Chem. Soc. 70, 1510 (1948); coauthor: R. Livingston.
 “Absorption Spectra of Solutions of Pheophytin a in Methanol Containing Acid or Base.“ J. Am. Chem. Soc. 75, 3025 (1953); coauthors: R. Livingston, L. Thompson, A. Weiler.
 “A Semi-Empirical Theory of the Electronic Spectra and Structure of Complex Unsaturated Molecules 1.“ J. Chem. Phys. 21, 466 (1953); coauthor: R.G. Parr.
 “An Improvement in die pi-Electron Approximation in LCAO MO Theory.“ J. Chem. Phys. 21, 568 (1953).
 “A Semi-Empirical Theory of die Electronic Spectra and Structure of Complex Unsaturated Molecules II.“ J. Chem. Phys. 21, 767 (1953).
 “On the Electronic Structure and Electronic Spectra of Ethylene-Like Molecules.“ J. Chem. Phys. 23, 711 (1955); coauthor: R.G. Parr.
 “The Pheophtin-Sensitized Photoreduction of p-Dimethylaminoazobenzene by Ascorbic Acid.“ J. Am. Chem. Soc. 78, 2944 (1956); coauthor: R. Livingston.
 “Some Photochemical Oxidation Reduction Reactions Sensitized by Chlorophyll a and by Pheophytin a.“ J. Am. Chem. Soc. 78, 2948 (1956); coauthor: R. Livingston.
 “Theory of the Electronic Spectra and Structure of the Polyacenes and of Alternate Hydrocarbons‘
J. Chem. Phys. 24, 250 (1956).
 “Electronic Spectrum and Structure of Azulene.“ J. Chem. Phys. 25, 1112 (1956).
 “The Dynamic Mechanical Properties of Polyhexene-1 .‚ J. Appl. Phys. 28, 499 (1957); coauthors: S.F. Kurath, E. Passaglia.
 “The Dynamic Mechanical Properties of ‘Hypalon‘-20 Synthetic Rubber at Small Strains.“ J. Appl.Polym. Sci. 1, 150—157 (1959); coauthors: S.F. Kurath, E. Passaglia.
 “A New Apparatus for Determining the Cell Structure of Cellulose Materials.“ Rubber World 138, 261 (1958); coauthor: W.J. Remington.
 “A New Method for Measunng the Degree of Crosslinking in Elastomers.“ J. Polym. Sci. XLV, 341 (1960); coauthors: E. E Cluff, E. K. Gladding.
 “Neuere Ergebnisse zur Elastomeren-Vernetzung.“ Kunststoffe 50, 623 (1960); English: German Plastics Digest 50 (11), 7 (1960).
 “Dynamic Mechanical Properties of Neoprene Type W.“ (Contr. No. 114) J. Appl. Polym. Sci. 7, 667 (1963); coauthor: T. P. Yin.
 “Consumption of Mercaptan Chain Transfer Agents in Chloroprene Polymerization.“ J. Appl. Polym. Sci. 7, 675 (1963); coauthor: J. W. McFarland (Contr. No. 115).
 “Copolymerization as a Markov Chain.“ J. Chem. Phys. 39, 2303 (1963); coauthor: H. K. Frensdorff.
 “Dynamic Mechanical Properties of Several Elastomers and the Potentialities in Vibration Control Applications.“ J. Appl. Polym. Sci. 8, 2427 (1964); coauthor: 1. P. Yin.
 “The Frequency-Temperature Dependence of the Damping Characteristics of Several Elastomers.“
Presented at the Mare Island Shipyard, Vallejo, California, Structural Vibration Damping Confer-
ence; coauthor: T. P. Yin (Contr. No. 121).
 “Cis-Polychloroprene.“ J. Polym. Sci. Part A. 2, 4727 (1964); coauthor: C. A. Aufdermarsh, Jr.
 “Prediction of Molecular Weights for Condensation Polymerization of Polyfunctional Monomers.“
95 (1972); coauthors: K. D. Ziegel, A. W. Fogiel.
 “Elastomers and Flammability.“ Rubber Age 107, 29 (1975); coauthors: P. R. Johnson, J. J. McEvoy
(Contr. No. 315).
 “Thermoplastic Elastomers Based on Polypivalolactone Graft Systems.“ (l)Published in proceedings of Polymer Colloquium in Kyoto, Japan September 1977; (2) Published in proceedings of US-Japan Seminar in Akron for Elastomers at the University of Akron, October 17—21, 1977.
 “Fluoropolymers: Their Development and Performance.“ Proceedings ofthe Robert A. Welch Foundation Conf. on Chem. Res. XXVI (1983); coauthors: D. C. England, R. E. Uschold, H. Starkweather.
 “Value Added Products.“ CHEMRAWN III World Conference on Resource Material Conversion: (Bio-)Chemical Process Bridges to Meet Future Needs, The Hague, 1984. Proceedings of the Conference; coauthor: H. J. Glidden.
 “Polymer Science and Engineenng: Challenges, Needs and Opportunities.“ National Academy Press, 1981. Cochairman with C. G. Overberger of NRC Panel on Polymer Science and Engineering.
 “High Performance Polymer Composites.“ Research Briefings 1984, National Academy Press, 1984. Cochairman with David McCall of NAS/NAE Panel on High Performance Composites.
 “Polymides: Versatile Specialty Polymers,“ Polym. 1. 19, 127 (1987).
 “Industrial Corporate Research: Perspectives on Innovation.“ Innovation at the Crossroads between Science and Technology, The S. Neaman Press, Technion City, Haifa, Israel, p. 185, 1989.
Recent Plenary and Invited Lectures
“Challenges, Needs and Opportunities in Polymer Science and Engineering,“ The Society of Polymer Science, Japan, Commemorating die 3Oth Anniversasy, Nagoya, Japan, October 18, 1982.
“New Orientations for Polymer Science and Technology,“ The Israel Chemical Society SOth Anniversary,Jerusalem, Israel, April 12, 1984.
“Control of Structure in Polymenc Systems,“ Japan-U.S. Polymer Symposium, Kyoto, Japan, October 28, 1985.
“Polymers: Reality and Dreams,“ AAAS National Meeting, Philadelphia, Pennsylvania, May 27, 1986.
“Polyimides: Versatile Specialty Polymers,“ 2nd SPSJ International Polymer Conference, Tokyo, Japan, August 21, 1986.
“Polymer Education from the Perspective of Industry,“ Paul J. Flory Symposium, National ACS Meeting, Anahelm, California, September 11, 1986.
“Molecular Engineering and Overlap (Revisited),“ Ernest L. Eliel and Robert G. Parr Symposium, University of North Carolina, Chapel Hill, North Carolina, March 20, 1987.
“Composites: Perspectives and Recommendations,“ IUPAC CHEMRAWN VI, World Conference on Advanced Materials, Tokyo, Japan, May 21, 1987.
“Advanced Materials Science: The Overlaps,“ Asilomar Polymer Conference, Monterey, California, February 2l, 1988.
“Industrial Corporate Research: Perspectives on Innovation,“ International Workshop on Innovation, The Samuel Neuman Institute and the Von Len Institute, Jerusalem, Israel, May 25, 1988.
“Materials Science in Du Pont,“ Advanced Materials Conference celebrating the SOth anniversary of Nylon and Teflon, Wilmington, DE, October 24, 1988.
“Multidiscipliuary Aspects of Advanced Materials Development,“ Conference on High technology Materials, Virginia Commonwealth University, Richmond, VA, January 6, 1989.
“Directions in Polymer Science,“ Workshop on Chemical Research .Frontiers, cosponsored by national academies of Mexico and U.S.A., Tequesquitengo, Mexico, May 24, 1989.
“Whither Polymer Research,“ Polymers Gordon Conference, New London, NH, June 29, 1989.
U.S. 3,265,672 — Pariser, Pattison, & Seligman—Case 1
Issued 8/9/66 — “Elastomeric Copolymers of Ethylene, An Alkyl Vinyl Sulfide, and Optionally Minor Proportions of Acrylic Acid or Methacrylic Acid, or Selected Derivatives Thereof“
U.S. 3,035,012 — Pariser & Perkins—Case 1
Issued 5/15/62 — “Graft Copolymers of Chloroprene and
U.S. 3,042,652 — Pariser & Souffle—Case 1
Issued 7/3/62 — “Blends Containing Iffadiated Polychloroprene“
U.S. 3,385‚833 — Pariser & Witsiepe—Case 1
Issued 5/28/68 — “Improved Process for Manufacture of Thermoplastic Polyurethane Elastomers“
U.S. 3,147,317 — Jungk & Pariser—Case 1
Issued 9/1/64 — “Easily Processable Blends of Chloroprene Polymers“
Received June 26, 1989
Accepted for publication August 21, 1989
Last updated : Feb. 17, 2002 - 19:30 CET