The Pariser-Parr-Pople PPP Method

The two-electron integrals are evaluated by applying the ZDO approximation (above) to all different orbitals r and s  

3 The Complete Neglect of Differential Overlap (CNDO) Method 

There are two versions of CNDO, CNDO/1 and an improved version, CNDO/2. First look at CNDO/1. Consider the core integrals Hcrwhere both orbitals are the same (i.e. the same orbital occurs twice in the integral ( / r(l) /7J °re / ,.(l))) and are on the same atom A. Recall the example of an ab initio calculation on HHe+ (Section 5.2.36e). Consider, say, element (1,1) of that Hcore matrix. From Eq. 5.116  

Equation 6.9 can be generalized to a matrix element (r,r) and a molecule with 

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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... 

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... 

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). 

A thione-thiol structure in the crystalline state and in aprotic solvents was established for the [l,2,4]triazolo[3,4- >][l,3,4]thiadiazole (37), in accordance with calculations of the 7r-bonding and solvation energies by the Pariser-Parr-Pople (PPP) method and also on the basis of IR and UV spectral information (75CHE304). The electron-density distribution in four tautomeric forms, calculated by the PPP method, also indicates that the ring-junction carbon atom is the most reactive one with respect to nucleophilic attack (75CHE500). 

The eigenvalues of the Hamiltonian matrix obtained by the Pariser-Parr-Pople (PPP) method. 

The ground-state dipole moments of BPHTs were calculated by evaluating the vector sum of the it moment (computed by the Pariser-Parr-Pople (PPP) method) and the a moment (from cr-bond moments) [17], a separate set of dipole moment values being obtained by the parametric method 3 (PM3) ([25] and references therein) (Table 5). By comparison, the calculated ground-state dipole moment values were considerably lower than the experimental ones. In some cases, the agreement between the experimental and calculated ground-state dipole moments was rather poor [17]. 

Hiickel-type methods include only one-electron integrals and are therefore noniterative. Two-electron interactions are taken into account explicitly in semiempirical self-consistent-field (SCF) methods. Again, the first such approaches were restricted to ir-electrons, most notably the Pariser-Parr-Pople (PPP) method (1953)... 

A semiempirical 7r-electron theory that takes electron repnlsion into acconnt and thereby improves on the Hiickel method is the Pariser-Parr-Pople (PPP) method, developed in 1953. Here, the 7r-electron Hamiltonian (17.1) inclnding electron repnlsions is nsed, and the 7r-electron wave fnnction is written as an antisymmetrized product of TT-electron spin-orbitals. A minimal basis set of one Iprr STO on each conjngated atom is used, and the spatial tt MOs are taken as linear combinations of these AOs 4> = 2r=i Crifr- The Roothaan eqnations are used to find SCE tt MOs within the TT-electron approximation. 

What has come to be called the Pariser—Parr— Pople (PPP) method is an amalgam of the. Tr-electron SCF treatment of Pople with his point charge integral approjdmations replaced by the parameter values of Pariser and Parr. In 1965 Chung and Dewar examined the annulenes using the Huckel, the PPP, and Dewar s own split p-orbital (SPO) method to compute ji energies. ... 

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