Neglect of Differential Overlap NDO

NBO (natural bond order) the name of a set of population analysis techniques NDO (neglect of differential overlap) the fundamental assumption behind many semiempirical methods... 

Parr ° and by Pople (PPP) to explicitly include self-consistent coulombic electron repulsion. A second development, crucial for magnetic properties, was the inclusion of a-valence elearons in the late 1950s and early 1960s. This period gave birth to the extended Hiickel theory (EHT), which has proved extremely valuable for the analysis of ground-state reaaivities, and also to CNDO/1, the first in a series of all valence electron NDO (neglect of differential overlap) methods. 

The complete neglect of differential overlap (CNDO) method is the simplest of the neglect of differential overlap (NDO) methods. This method models valence orbitals only using a minimal basis set of Slater type orbitals. The CNDO method has proven useful for some hydrocarbon results but little else. CNDO is still sometimes used to generate the initial guess for ah initio calculations on hydrocarbons. 

The Extended Hiickel method neglects all electron-electron interactions. More accurate calculations are possible with HyperChem by using methods that neglect some, but not all, of the electron-electron interactions. These methods are called Neglect of Differential Overlap or NDO methods. In some parts of the calculation they neglect the effects of any overlap density between atomic orbitals. This reduces the number of electron-electron interaction integrals to calculate, which would otherwise be too time-consuming for all but the smallest molecules. 

The most elementary all valence electron NDO model is that known as Ippmplete neglect of differential overlap (CNDO). Segal and Pople introduced (his in 1966. Only valence electrons are explicitly treated, the inner shells being tijicen as part of the atomic core. The ZDO approximation is applied to the WO-electron integrals, so that... 

Most present-day semiempirical methods are based on the idea of the neglect of differential overlap (NDO) of inner electrons developed by Pople and co-workers (see, for example, Pople and Beveridge, 1970 Dewar, 1975). NDO-type approximations generally result in a decrease in computational resource requirements that are 1/100 to 1/1000 of the corresponding ab initio methods. 

Surprisingly for such a conceptually simple method, the EHM has a theoretically-based advantage over otherwise more elaborate semiempirical methods like AMI and PM3, in that it treats orbital overlap properly those other methods use the neglect of differential overlap or NDO approximation (Section 6.2), meaning that they take Stj = f),r as in the simple Hiickel method. This can lead to superior results from the EHM [66]. 

The basic NDO approximation was developed by Pople and is known as the Complete Neglect of Differential Overlap CNDO semiempirical method (Pople et al. 1965 ... 

Semiempirical methods that also include the a electrons have been developed and are referred to as INDO and CNDO. NDO stands for Neglect of Differential Overlap. We will not treat these approximations here. C in CNDO stands for Complete and I in INDO stands for Intermediate. Approximations where /S is added at the end mean that the parametrization is relevant for the description of excited states and that Cl is used. AMI (Austin model 1) and PM3 (Parametrized model 3) are more recent models that build on the NDO approximation and are still in use. The AMI (developed by M. J. S. Dewar) and PM3 (developed by J. J. P. Stewart) methods are useful to get quick results on comparatively large systems. PM3 in particular has been extensively used in this book, mainly for illustration purposes. 

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