MNDOC method

A variation on MNDO is MNDO/d. This is an equivalent formulation including d orbitals. This improves predicted geometry of hypervalent molecules. This method is sometimes used for modeling transition metal systems, but its accuracy is highly dependent on the individual system being studied. There is also a MNDOC method that includes electron correlation. 

W. Thiel, The MNDOC method, a correlated version ofthe MNDO model, J. Am. Chem. Soc. 103, 1413-1420(1981). 

As was mentioned in the introduaion, a comprehensive survey of semiempirical methods and MNDO in particular has recently been written by Thiel. In it he rigorously describes the reladonship between semiempirical and ab initio methods. His survey summarizes the applicadons of MNDO to the study of chemical reaction. Thiel is the author of the MNDOC method, a correlated form of MNDO he describes the relationship of this method to other semiempirical methods. Since his survey includes very effecdvely the current status of semiempirical mediods, a similar review is redundant, and interested readers are referred to Thiel s work. 

Mixed basis sets, 154 MM2, MM3, MM4 force fields, 40 MMFF force field, 40 MMP2, MMP3 method, 27 MNDO/d method, 89 MNDOC method, 87... 

The semiempirical molecular orbital (MO) methods of quantum chemistry [1-12] are widely used in computational studies of large molecules. A number of such methods are available for calculating thermochemical properties of ground state molecules in the gas phase, including MNDO [13], MNDOC [14], MNDO/d [15-18], AMI [19], PM3 [20], SAMI [21,22], OM1 [23], OM2 [24,25] MINDO/3 [26], SINDOl [27,28], and MSINDO [29-31]. MNDO, AMI, and PM3 are widely distributed in a number of software packages, and they are probably the most popular semiempirical methods for thermochemical calculations. We shall therefore concentrate on these methods, but shall also address other NDDO-based approaches with orthogonalization corrections [23-25],... 

The first (1967) of the Dewar-type methods was PNDDO [35], partial NDDO), but because further development of the NDDO approach turned out to be unexpectedly formidable [33], Dewar s group temporarily turned to INDO, creating MINDO/1 [36] (modified INDO, version 1). The third version of this method, MINDO/3, was said [33] [to have] so far survived every test without serious failure , and it became the first widely-used Dewar-type method. Keeping their promise to return to NDDO the group soon came up with MNDO (modified NDDO). MINDO/3 was made essentially obsolete by MNDO, except perhaps for the study of carbocations (Clark has summarized the strengths and weaknesses of MINDO/3, and the early work on MNDO [37]). MNDO (and MNDOC and MNDO/d) and its descendants, the very popular AMI and PM3, are discussed below. Briefly mentioned are a modification of AMI called SAMI and an... 

Schroder and Thiel have compared MNDO (Section 6.2.5.3) and MNDOC (Section 6.2.5.7) with ab initio calculations for the study of the geometries and energies of 47 transition states [47]. AMI and PM3 calculations should give somewhat better results than MNDO for these systems, since these two methods... 

Tel. 913-268-3271, fax 913-268-3445, e-mail aholder vaxl.umkc.edu Semiempirical molecular orbital calculations with M. J. S. Dewar s SAMI parameterization, including d orbitals for transition metals, and a graphical user interface. MINDO/3, MNDO, MNDOC, AMI, and PM3 methods. DEC, Cray, Silicon Graphics, Sun, and PCs (under X-Windows). 

These results also follow from the cross sections through the potential energy surfaces calculated in the semiempirical all-valence electron approximation by the MNDOC-CI method for acetaldehyde, acrolein, and benzal-dehyde, which are displayed in Figure 7.13. Stabilization of the state... 

MNDO was also parameterized at the correlated level by Thiel this work has recently been reviewed in the context of other methods. The basic model is the same as that described for MNDO, but some of the parameters in the MNDOC program were changed to obtain better geometries and heats of formation. 

M. J. S. Dewar and W. Thiel, / Am. Chem. Soc., 99, 4899 (1977). Ground States of Molecules. 38. The MNDO Method. Approximations and Parameters. The MNDOC program is readily available W. Thiel, QCPE Bull., 2, 63 (1982). QCPE Program 438, MNDOC Correlated Semiempirical Calculations with Geometry Optimization. 

The comparison by Schroder and Thiel [62] (section 6.3.1) of SE (MNDO and MNDOC) and ab initio geometries and energies conclnded that the SE methods usually overestimate activation energies. Of 21 activation energies (Table IV in Ref. [62], entries I, K, W omitted), MNDO overestimated (compared with best correlated ab initio calcnlations) 19 and nnderestimated 2 the overestimates ranged from... 

Based on the work of Thiel, such a semiempirical MR-CI was developed some time ago. This MNDOC-CI method may be characterized as follows. For each state under consideration, one or a few reference configurations are used, and from these reference configurations single and double excitations within a properly chosen active space (AS) are considered in constructing spin-adapted configuration state functions (CSFs). To specify the active space and the refer-... 

Starting with y= 120° and 0 = 0°, the MNDOC-CI method yields a conical intersection, the structure of which is shown in Figure 6.8. The values 0=0° for the rotational angle and 7 = 143,7° for the bond angle are as expected the bond distances are between those of azirine and nitrile ylide, especially because the distance between nitrogen and the methylene carbon r N = 1.34 A has decreased and is equal to the double-bond value r N = 1.34 A in the nitrile ylide. To completely establish the mechanism, it remains to be shown that the conical intersection is accessible without a barrier from the Franck-Condon geometry and that the nitrile ylide can be reached from the conical intersection. This is easily... 

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