Pariser–Parr–Pople method

The Pariser-Parr-Pople applies semiempirical quantum mechanical methods to the quantitative prediction of electronic structure and spectra in molecules of interest to chemists. Previous methods existed, such as Hückel's rule, but were limited in their scope, application and complexity.

This approach was developed in the 1950s by Rudolph Pariser with Robert Parr and co-developed by John Pople. It is essentially a more efficient method for coming up with reasonable approximations of molecular orbitals. Molecular orbital characteristics have implications with regards to both the basic structure and reactivity of a molecule. This method used a Zero Differential Overlap (ZDO) approximation to reduce the problem to reasonable size and complexity but still required modern solid state computers (as opposed to puch-card or vacuum tube systems) before becoming useful for molecules larger than benzene.

Interestingly enough, Pariser's goal of using this method to predict the characteristics of complex oganic dyes was never realized, though the method had wide applicability in precise prediction of electronic transitions, particularly lower 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.