Editing Pariser–Parr–Pople method

{{Short description|Calculation method in quantum chemistry}}
In [[molecular physics]], the '''Pariser–Parr–Pople method''' applies [[Computational Chemistry#Semiempirical methods|semi-empirical]] [[quantum mechanical]] methods to the quantitative prediction of [[electronic structure]]s and spectra, in [[molecule]]s of interest in the field of [[organic chemistry]]. Previous methods existed&mdash;such as the [[Hückel method]] which led to [[Hückel's rule]]&mdash;but were limited in their scope, application and complexity, as is the [[Extended Hückel method]].

This approach was developed in the 1950s by [[Rudolph Pariser]] with [[Robert Parr]] and co-developed by [[John Pople]].<ref>R. Pariser and R. Parr, [[Journal of Chemical Physics]], 21, 466, (1953)</ref><ref>R. Pariser and R. Parr, [[Journal of Chemical Physics]], 21, 767 (1953)</ref>
<ref>J. A. Pople, [[Transactions of the Faraday Society]], 49, 1375, (1953)</ref> It is essentially a more efficient method of finding reasonable approximations of [[molecular orbital]]s, useful in predicting physical and chemical nature of the molecule under study since molecular orbital characteristics have implications with regards to both the basic [[Chemical structure|structure]] and [[reactivity (chemistry)|reactivity]] of a molecule. This method used the [[zero-differential overlap]] (ZDO) approximation to reduce the problem to reasonable size and complexity but still required modern [[solid state (electronics)|solid state]] [[computer]]s (as opposed to [[punched card]] or [[vacuum tube]] systems) before becoming fully useful for molecules larger than [[benzene]].

Originally, Pariser's goal of using this method was to predict the characteristics of complex organic dyes, but this was never realized. The method has wide applicability in precise prediction of electronic transitions, particularly lower [[Singlet state|singlet]] transitions, and found wide application in theoretical and applied [[quantum chemistry]]. The two basic papers on this subject were among the top five chemistry and physics citations reported in ISI, Current Contents 1977 for the period of 1961–1977 with a total of 2450 references.

In contrast to the [[Hartree&ndash;Fock]]-based  [[Semi-empirical quantum chemistry methods|semiempirical]] method counterparts (i.e.: [[MOPAC]]), the pi-electron  theories have a very strong [[ab initio]] basis.  The PPP formulation is actually an approximate pi-electron effective operator, and the empirical parameters, in fact, include effective [[electron correlation]] effects. A rigorous,  ab initio theory of the PPP method is provided by diagrammatic, multi-reference, high order perturbation theory (Freed, Brandow, Lindgren, etc.).  (The exact formulation is non-trivial, and requires some field theory) Large scale ab initio calculations (Martin and Birge, Martin and Freed, Sheppard and Freed, etc.) have confirmed many of the approximations of the PPP model and explain why the PPP-like models work so well with such a simple formulation.

==References==
<references/>

{{DEFAULTSORT:Pariser-Parr-Pople method}}
[[Category:Molecular physics]]
[[Category:Semiempirical quantum chemistry methods]]


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