Semiempirical methods Differential Overlap

The PRDDO (partial retention of diatomic differential overlap) method is an attempt to get the optimal ratio of accuracy to CPU time. It has been parameterized for the periodic elements through Br, including the 3rd row transition metals. It was parameterized to reproduce ah initio results. PRDDO has been used primarily for inorganic compounds, organometallics, solid-state calculations, and polymer modeling. This method has seen less use than other methods of similar accuracy mostly due to the fact that it has not been incorporated into the most widely used semiempirical software. 

The typed neglect of differential overlap (TNDO) method is a semiempirical method parameterized specifically to reproduce NMR chemical shifts. This... 

There is one semiempirical program, called HyperNMR, that computes NMR chemical shifts. This program goes one step further than other semiempiricals by defining different parameters for the various hybridizations, such as sp carbon vs. sp carbon. This method is called the typed neglect of differential overlap method (TNDO/1 and TNDO/2). As with any semiempirical method, the results are better for species with functional groups similar to those in the set of molecules used to parameterize the method. 

MINDO (modified intermediate neglect of differential overlap) a semiempirical method... 

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... 

A method which is similar to the Pariser-Parr-Pople method for the n electron system and is applicable to common, saturated molecules has been proposed by Pople 28>. This method is called the CNDO complete neglect of differential overlap) SCF calculation. Katagiri and Sandorfy 29> and Imamura et al. °) have used hybridized orbitals as basis of the Pariser-Parr-Pople type semiempirical SCF calculation. 

Semiempirical methods are widely used, based on zero differential overlap (ZDO) approximations which assume that the products of two different basis functions for the same electron, related to different atoms, are equal to zero [21]. The use of semiempirical methods, like MNDO, ZINDO, etc., reduces the calculations to about integrals. This approach, however, causes certain errors that should be compensated by assigning empirical parameters to the integrals. The limited sets of parameters available, in particular for transition metals, make the semiempirical methods of limited use. Moreover, for TM systems the self-consistent field (SCF) procedures are hardly convergent because atoms with partly filled d shells have many... 

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. 

A number of papers have appeared recently in which semiempirical quantum mechanical methods, such as the complete neglect of differential overlap (CNDO), incomplete neglect of differential overlap (INDO), or Hiickel methods, have been applied to electron-deficient systems in an attempt to calculate their properties (31, 49, 64, 75, 77, 78, 89, 90, 92). Although the quantitative results of these calculations must be treated with great care, they do provide an indication of some of the parameters that determine formation and stability of electron-deficient bonded systems. 

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]. 

Semiempirical methods differ amongst themselves in, amongst other ways, the criteria for setting dS = 0, i.e. for applying zero differential overlap, ZDO. 

As early as in 1972, a theoretical study of the structure of unknown 1,2,3,4-thiatriazole was carried out using the semiempirical complete neglect of differential overlap (CNDO) method <1972JCD73>. Because of the known large discrepancy for the C-S length predicted by semi-empirical calculations compared with the experimental data for l,2,3,4-thiatriazole-5-thiolate <1986ICA71>, this study has only historical interest. 

One semiempirical method which has been a workhorse in studies of fullerenes is the MNDO (Modified Neglect of Differential Overlap) method [4], which treats all valence electrons explicitly but within an essentially molecular... 

ZINDO57-58 is a semiempirical intermediate neglect of differential overlap/spectroscopy (INDO/S) based routine. It can be combined with an SOS method to calculate second-order nonlinear optical coefficients. ZINDO is parametrized to accommodate transition-metal calculations and is therefore suited for calculation on organometallic compounds. To achieve computational efficiency, some of the terms in Eq. (2) are replaced by empirical data or neglected. To see how the INDO/S does this, the closed-shell case will be examined.57 58 It is useful to introduce the following ... 

The semiempirical quantum mechanical approximation82-86 that has been called CNDO/2 (for complete neglect of differential overlap) was used to calculate binding energies in the trigonal bipyramidal configuration and in a series of model situations that are pertinent to the TR and the BPR mechanisms in a variety of molecules.81 This approximation to the solution of the LCAO-SCF equations yields results that provide, at least, a trend in the real systems involved. It has been shown that, with reasonable approximations and parametrization, this method offers an adequate compromise with respect to the more rigorous and accurate but far more laborious and expensive ah initio methods.82-85... 

The second group of semiempirical MO LCAO methods is constituted by zero-differential overlap methods (9). Two subgroups can be specified here which somewhat conventionally may be called physical and chemical. The former involves CNDO/2, INDO, and some of their modifications, for example, CNDO/S. These methods are directed to the calculations of the electron characteristics charge distributions, dipole moments, polarizabili-... 

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