INDO approximation

MIXDO/3 is the earliest of the Dewar methods. It provides more accurate geometries and heats of formation than CNDO or INDO. and has been used widely. The limitations of the INDO approximation, on which MI lhO/3 is based, frequently lead to problems of accuracy wdi cri dealing w i th m olecules con tain ing h eteroatorn s. 

Using the same nomenclature as for the INDO approximation, the elements of the MfNDOfS UHFFock matrix are described below. When < )j an d d tti iJ on different cen lers the off-diagonal elem en ts are... 

A number of MO calculations has been carried out, and these have had mixed success in predicting chemical reactivity or spectroscopic parameters such as NMR chemical shifts and coupling constants. Most early calculations did not take into account the contribution of the sulfur 3d-orbitals to the ground state, and this accounts for some of the discrepancies between calculations and experimental observations. Of the MO methods used, CNDO/2 and CNDO/S have been most successful the INDO approximation cannot be used because of the presence of the sulfur atom. 

Using the INDO approximation on fully optimized geometries, the lone pair orbital energy (eN) (of 3-N) in 4-aminoimidazole (179 R = R2 = H) was calculated to be 0.4482 au and the protonation energy (AEp) of the same molecule was calculated to be 374.9 kcal mol-1 (83H1717). 

MNDO. Despite its success, Dewar recognized certain weaknesses (6) in MINDO/3 due to the INDO approximation, such as the inability to model lone pair - lone pair interactions. Additionally, due to the use of diatomic parameters in MINDO/3, it was increasingly difficult to extend MINDO/3 to additional elements. Because of this, Dewar began working on a new model based on the better NDDO approximation. 

On the other hand, for studying spin - spin couplings in medium-sized molecules it is common to use the intermediate neglect of differential overlap77 (INDO) approximation, which, although it does not permit accurate calculation, still gives an adequate interpretation of many experimental trends.74,77... 

The excellent correlation between E m of 2-substitued 5(6)-nitrobenzimidazoles and their ELUM0 (INDO approximation) [884] points out to a linear dependence between these sizes (3.6) [991] ... 

MO calculations (H.) in the INDO approximation indicate that the unpaired electron occupies a totally symmetric antibonding... 

CNDO/S, CNDO/FK, CNDO/BW, INDO/1, INDO/2, INDO/S and SINDOl. These methods are rarely used in modem computational chemistry, mainly because the modified methods described below usually perform better. Exceptions are INDO based methods, such as SlNDOl and INDO/S. SINDO Symmetric orthogonalized INDO) employes the INDO approximations described above, but not the ZDO approximation for the overlap matrix. The INDO/S method (INDO parameterized for Spectmscopy) is especially designed for calculating electronic spectra of large molecules or systems involving heavy atoms. 

MINDO/3 is based on the INDO approximation. The remaining theories of this subsection are based on the far more justifiable NDDO approximation (see the preceding subsection). Tliese NDDO-based Dewar-type theories give significantly better results than MINDO/3, and MINDO/3 is essentiaUy obsolete, being little used nowadays. 

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