Pariser-Parr-Pople method,

The Pariser-Pople-Parr method is the simplest implementation of the self-consistent field method. Like the Hiickel method, it treats only the electrons in delocalized pi orbitals in planar molecules and represents these orbitals as linear combinations of basis functions that include only the unhybridized p orbitals. All overlap integrals are assumed to vanish, as in the Hiickel method, but not all of the integrals representing electron-electron repulsions are assumed to vanish. Two notations are used for these integrals ... 

The integrals are again parameterized as in eq. (3.79). The approximations for the one-electron integrals in CNDO are the same as for INDO. The Pariser-Pople-Parr (PPP) method can be considered as a CNDO approximation where only 7r-electrons are treated. 

The Pariser-Pople-Parr (PPP) method is a special case of CNDO, restricted to the treatment of n electrons. 

In view of the inadequacy of the HMO method, attention turned in time to rather more elaborate MO approaches that remained reliant on semi-empiricism. A particular molecular orbital approach that was extensively applied to the calculation of the colour properties of dye molecules in the later decades of the twentieth century is the Pariser-Pople-Parr (PPP) method. Although the use of the PPP-MO method... 

Semiempirical ir-orbitals methods (Pariser-Pople-Parr PPP)... 

H = Hiickel, EH = extended Hiickel, PPP = Pariser-Pople-Parr, CNDO = complete neglect of differential overlap, INDO = intermediate neglect of differential overlap, NDDO = neglect of diatomic differential overlap, MNDO = modified neglect of differential overlap, MINDO = modified intermediate neglect of differential overlap. Note AMI and PM3 are MNDO methods that differ only in the way constants are chosen to approximate various integrals. 

The Pariser-Parr-Pople (PPP) method is an extension of the Huckel method that allows heteroatoms other than hydrogen. It is still occasionally used when... 

Some useful, conventional SCF methods have been proposed by Pople 21) and by Kon 22> using the semiempirical calculation of Pariser and Parr 23> with regard to the it electrons of planar conjugated molecules. 

The Pariser-Parr-Pople semiempirical method has been applied to dibenzothiophene to predict bond orders, relative electro-... 

Considering the proliferation of MO calculations in recent years, it is remarkable how few calculations have been reported for these heterocycles. Frontier orbitals of several betaines have been calculated using the Hiickel method and electroselectivity correctly predicted on the basis of the orbital symmetry.Similar results have been obtained for pyridinium-3-olates (427) using the Pariser- Parr Pople (PPP) method. The CNDO... 

Much work has been reported on type B azapentalenes, and this can be classified according to the degree of sophistication of the calculations. Simple Hiickel linear combination of atomic orbitals (LCAO) methods have been widely used, though more recently more sophisticated Pariser-Parr-Pople (PPP) methods (using only re-electrons) and all-electron CNDO calculations have been reported. 

A semi-empirical ir-electron theory that takes electron repulsion into account is the Pariser-Parr-Pople (PPP) method (R.G. Parr. Quantum Theory of Molecular Electronic Structure, Benjamin, N.Y., 1963). 

Under some simplifications associated with the symmetry of fullerenes, it has been possible to perform calculations of type Hartree-Fock in which the interelec-tronic correlation has been included up to second order Mpller-Plesset (Moller et al. 1934 Purcell 1979 Cioslowski 1995), and calculations based on the density functional (Pople et al. 1976). However, given the difficulties faced by ab initio computations when all the electrons of these large molecules are taken into account, other semiempirical methods of the Hiickel type or tight-binding (Haddon 1992) models have been developed to determine the electronic structure of C60 (Cioslowski 1995 Lin and Nori 1996) and associated properties like polarizabilities (Bonin and Kresin 1997 Rubio et al. 1993) hyperpolarizabilities (Fanti et al. 1995) plasmon excitations (Bertsch et al. 1991) etc. These semiempirical models reproduce the order of monoelectronic levels close to the Fermi level. Other more sophisticated semiempirical models, like the PPP (Pariser-Parr-Pople) (Pariser and Parr 1953 Pople 1953) obtain better quantitative results when compared with photoemission experiments (Savage 1975). 

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