Quantum Semiempirical Picture

Based on the idea of orbital electronegativity introduced by Hinze-Whitehead-Jaffe, in 1981 Ponec had introduced the orbital electronegativity based on the semiempirical approximation CNDO ( complete neglecting differential overlapping , the complete neglect of the differential-orbital overlap even for the orbitals belonging to the same atom) (Ponec, 1981). 

in the limits CNDO, which essentially means to consider only the diagonal integrals, the configuration energy of an atom A is written as  

Quantum Nanochemistry—Volume II Quantum Atoms and Periodicity 

Using the expression of CNDO energy is directly calculated the ionization energy and the affinity for the electrons of the orbitals of atom A, 

the global electronegativity of the atom A in molecule can be evaluated as  

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The system under study is assumed to consist of 2A, electrons, possibly in the presence of a nuclear framework. An orbital picture of the quantum behaviour of the system is then introduced on accepting the validity of an independent-particle model where each electron moves in the field of an effective potential v(r), which afterwards is left substantially unspecified. We emphasize, however, that the choice of v(F) is an essential step of any modeling. Besides semiempirical forms, effective potentials v[ (r)] functionally dependent on the electron numeral density n(r) are intuitively bound to play a prominent role in applications. 

The topic of interactions between Lewis acids and bases could benefit from systematic ab initio quantum chemical calculations of gas phase (two molecule) studies, for which there is a substantial body of experimental data available for comparison. Similar computations could be carried out in the presence of a dielectric medium. In addition, assemblages of molecules, for example a test acid in the presence of many solvent molecules, could be carried out with semiempirical quantum mechanics using, for example, a commercial package. This type of neutral molecule interaction study could then be enlarged in scope to determine the effects of ion-molecule interactions by way of quantum mechanical computations in a dielectric medium in solutions of low ionic strength. This approach could bring considerable order and a more convincing picture of Lewis acid base theory than the mixed spectroscopic (molecular) parameters in interactive media and the purely macroscopic (thermodynamic and kinetic) parameters in different and varied media or perturbation theory applied to the semiempirical molecular orbital or valence bond approach [11 and references therein]. 

Quantum-chemical ab initio studies have been performed on triads" " on tetrads " and on a pentad structure. Except for one recent paper we are not aware of further semiempirical calculations on base polyads. Even though semiempirical approaches 5deld occasionally sensible results we know fi om studies on nucleic acid base pairs that a consistent theoretical picture of their structure and energetics did only emerge when ab initio calculations with a consistent treatmait of electron correlation have become feasible. Hence, it is hi y recommraided to adopt non-empirical approaches for studies on base polyads. A detailed comparison of non-empirical, semiempirical and empirical approaches for the description of DNA base interactions has been performed by Hobza et al. ... 

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