NDDO methods

MTNDO/3 has been parameterized for H, B, C, N, O, F, Si, P, S and Cl, although certain combinations of these elements have been omitted. MINDO/3 is rarely used in modern computational chemistry, having been succeeded in accuracy by the NDDO methods below. Since there are parameters in MINDO which depend on two atoms, the number of parameters increases as the square of the number of elements. It is unlikely that MINDO will be parameterized beyond the above-mentioned in the future. 

Strengths of AMI. AMI is the first semiempirical NDDO method to accurately reproduce the heat of dimerization of water. This was achieved by modifying the core-core interaction. This change also corrected the instability of stericaUy crowded molecules, and the excessive stability of four membered rings. Average errors in AHf obtained using AMI are reduced by almost 40%, relative to MNDO. 

Strengths of PM3. PM3 is the first NDDO method to adequately treat hypervalent systems. This is exemplified by sulfuric add, for which the PM3 error in AHf is -5.8 Kcal/mol. On the whole, PM3 is more accurate than... 

Limitations of PM3. At the present time, PM3, the most recent NDDO method, has one severe limitation the lack of adequate testing. At present only information about ground state systems is available. 

Special parameterization of the NDDO method Parameterization of the NDDO method for hole transfer in DNA... 

To extend NDDO methods to elements having occupied valence d orbitals that participate in bonding, it is patently obvious that such orbitals need to be included in the formalism. However, to accurately model even non-metals from the third row and lower, particularly in hypervalent situations, d orbitals are tremendously helpful to the extent they increase the flexibility with which the wave function may be described. As already mentioned above, the d orbitals present in the SINDOl and INDO/S models make them extremely useful for spectroscopy. However, other approximations inherent in the INDO formalism make these models poor choices for geometry optimization, for instance. As a result, much effort over the last decade has gone into extending the NDDO fonnalism to include d orbitals. 

PNDO method, the F matrix is given, as in the NDDO method for closed shell systems, by... 

The trend is undoubtedly in favor of the development of an "allpurpose method, but the means by which this can be achieved are still debatable. Some authors believe that the direction to follow involves the development of an NDDO method. Such a procedure, however, would require the calculation of a much larger number of integrals and therefore would jeopardize the possibility of application to large organic molecules of. .chemical interest. ... 

The Neglect of Diatomic Differential Overlap (NDDO) Methods... 

NDDO [21] goes beyond INDO in that the ZDO approximation (Section 6.2.1, point (3)) is not applied to orbitals on the same atom, i.e. ZDO is used only for atomic orbitals on different atoms. NDDO is the basis of the currently popular semiempirical methods developed by M. J. S. Dewar and by coworkers who took up the torch MNDO, AMI and PM3 (as well as SAMI, PM5, and PM6). NDDO methods are the gold standard in general-purpose semiempirical methods, and the rest of this chapter concentrates on them. 

Was parameterized with data from over 9,000 compounds experimental and ab initio data were used, so unlike earlier NDDO methods (MNDO, AMI, PM3 ca. 1975-1990) the parameterization is not purely empirical. Only about 500 compounds were used for PM3. 

These remarks refer to NDDO methods like AMI and PM3. 

For PM5 (Stewart, 2002), no publication presenting the method is yet available. Interestingly, a recent comparative analysis of different NDDO methods reported that the PM5 mean absolute error in the heat of formation for a set of 622 compounds was slightly inferior to PM3, but superior to AMI (Repasky et al., 2002b). 

Semi-empirical NDDO methods have been used for the study of various heterocycles <1995JMT51>. A belief that highly accurate values are not essential for valid conclusions to be drawn and the time advantage of semi-empirical over ab initio methods were factors governing the choice of these procedures. In general, there is reasonable agreement between structures and energies computed and experimental or ab initio values. The semi-empirical structural data have been discussed and compared with previously determined structures (Table 1). 

With the NDDO methods, tautomeric equilibria,22o especially in heterocycles,216-219,223,224,227,232,233 have been a favorite topic for study using the BKO approach. The tautomeric equilibria of many heterocyclic systems are exquisitely sensitive to solvation,i i3>2i4 making them interesting test cases for the validation of any solvation model. A detailed comparison is presented later in the section on relative free energies in heterocyclic equilibria. A comprehensive study of the stabilization of a wide variety of carbon radical and ionic centers has also been reported.21 ... 

NDDO methods below. Since there are parameters in MINDO which depend on two ... 

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