PPP-CI method

In the first place the Hiickel approximation for the tt electrons has been replaced by a self-consistent field (SCF) procedure, generally in a semiempirical approximation of the Pariser-Parr-Pople type completed with some limited configuration interaction (PPP-CI method).59 00 Second, the a skeleton of the molecules has been treated by the Del Re procedure61 for saturated systems (which is the counterpart for the a electrons of the Hiickel method for 77 electrons) as refined for the a skeletons of conjugated heterocycles by... 

When parameters of the Pariser-Parr-Pople configuration interaction molecular orbital (PPP-CI MO) method were modified so as to reproduce the Aol)s values for l,3-di(5-aryl-l,3,4-oxadiazol-2-yl)benzenes 16 and 17, the calculated HOMO and LUMO energy levels corresponded with the experimental ionization potential and electron affinity values. The relationships between the electrical properties and molecular structures for the dyes were investigated. The absorption maximum wavelengths for amorphous films were found to be nearly equal to those for solution samples <1997PCA2350>. 

The electron densities, bond orders, first six excitation energies, oscillator strengths, and weighting factors of pyrido[l,2-f)]pyridazinium cation were calculated by the PPP semiempirical version of the SCFMO-CI method, which indicated that protonation is expected to take place at the nonbridgehead nitrogen, and nucleophilic substitution is predicted to occur at position 3 (68TCA417i... 

The latter three, (4)—(6), are utilized for qualitative elucidation of the observed ICD bands in signs, and the former three, (1)—(3), are successfully applicable for quantitative calculations of the observed ICD bands in both signs and magnitudes. The ICD of P-cyclodextrin complexes with benzene derivatives or azanaphthalenes162) has been analyzed by a molecular orbital calculation, using an approximation of PPP-type, which has been compared with the theoretical spectra calculated by using the CNDO/S-CI method on the basis of the MCD spectra 16S). 

Fig. 3.1 7r-Bond orders in the S0 (open circles) and T (filled circles) states of model polyenes with the number of conjugated double bonds, n = 5,7, 9, and 11 calculated by the PPP-SD-CI method, the details which are described in Ref. [4].

Favini applied the PPP method to vinyl azide. Here the delocalized tt system consists of 6 electrons and 5 atomic 2p orbitals. The calculated data include charge densities, bond orders and dipole moments (obtained by vector addition of the a and tt moments). Cis and trans isomers were considered... 

We decided to undertake this with the aim of formulating a generally applicable computational scheme for radicals which would be a natural extension of the PPP and semiempirical all-valence-electron methods for closed-shell molecules. We used for this purpose the self-consistent-field open-shell methods of Longuet-Higgins and Pople [2] and of Roothaan [3], we derived all expressions necessary for CI-S calculations [4, 5], and we tested the semiempirical open-shell PPP-type and INDO/S calculations systematically for various classes of radicals. 

Needless to say that after 30 years or so, this type of calculation, as all other semiempirical calculations, lost much of its importance. By means of highly sophisticated ab initio methods, such as SAC-CI [6] and CASPT2 [7], it is now possible to treat rather extensive open-shell systems and with remarkable accuracy. Still, we believe that for large conjugated hydrocarbon radicals, the open-shell PPP-type approach remains a method of choice. To document this we present a figure (Fig. 1) from our review on the radical anions of a, co-diphenylpolyenes. 

Until recently, most excited state calculations on large open-shell molecules made use of special versions of semiempirical methods such as PPP-Cl for planar n systems, " " " and CNDO/S or INDO/S-CP" " for molecules in which a electrons must be accounted for if excited states are to be adequately described. These semiempirical methods were parameterized for a Cl that includes only single excitations (CIS), which turns out to be sufficient for many applications. Although these methods have proven useful, their quantitative accuracy is limited, especially when it comes to modeling higher excited states, for which doubly excited configurations are frequently important. 

Since the establishment of necessary basis for accurate computation, simpler approaches with appropriate parameterization have been developed. In particular, parameterization of the PPP-SCF-CI (Pariser, Parr, Pople-self-consistent-field-configuration interaction) method has enabled /8-values (of sufficient accuracy to provide a basis for molecular design) to be obtained with much less powerful computers. Routines are now available for microcomputers that run in reasonable time with adequate accuracy [20]. 

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