Semiempirical MNDO, AMI, and

Tautomerism of 2,4-dihydroxyquinazoline was investigated using semiempirical MNDO, AMI, and PM3 methods (93KG1246). MNDO method erroneously indicated the predominance of the dihydroxy tautomer in the gas phase, whereas AMI and PM 3 methods predicted the predominance of the dioxo tautomer in agreement with the experimental findings. 

In ab initio methods the electrostatic term Ueie is the expectation value of the coulomb operator, as shown in Eq. (59). However, in the semiempirical MNDO, AMI, and... 

Dinadayalane, T. C., Sastry, G. N. (2002b). An assessment of semiempirical (MNDO, AMI and PM3) methods to model buckybowls. Journal of Molecular Structure (Theochem), 579, 63-72. 

The semiempirical techniques available include EH, CNDO, INDO, MINDO/3, ZINDO, MNDO, AMI, and PM3. The ZINDO/S, MNDO/d, and PM3(TM) variations are also available. The semiempirical module seems to be rather robust in that it did well on some technically difficult test calculations. 

The ah initio methods available are RHF, UHF, ROHE, GVB, MCSCF along with MP2 and Cl corrections to those wave functions. The MNDO, AMI, and PM3 semiempirical Hamiltonians are also available. Several methods for creating localized orbitals are available. 

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],... 

MNDO, AMI, and PM3 are based on the same semiempirical model [12, 13], and differ only in minor details of the implementation of the core-core repulsions. Their parameterization has focused mainly on heats of formation and geometries, with the use of ionization potentials and dipole moments as additional reference data. Given the larger number of adjustable parameters and the greater effort spent on their development, AMI and PM3 may be regarded as methods which attempt to explore the limits of the MNDO model through careful and extensive parameterization. 

The performance of the semiempirical methods for the calculation of thermochemical data depends on the extent to which the physics is included in the model and how well the neglected features can be accounted for by the parameterization. These methods can be assessed by validation against accurate experimental data or high level ab initio predictions. A summary of results for four semiempirical methods (MINDO/3, MNDO, AMI, and PM3) for the neutral enthalpies of formation in the G2/97 test set is given in Table 13. Overall, the newest method, PM3, does the best with an average absolute deviation of 7.02 kcal/mol. It has average absolute deviations of 3.91 and 4.27 kcal/mol for the subgroups of hydrocarbons and substituted hydrocarbons, respectively. 

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. 

A wide range of quantum-chemical methods have been used for studying properties of carbene analogs R2E (E = Ge, Sn, Pb) and various aspects of their chemistry. They include semiempirical methods (MNDO, AMI and PM3), ab initio calculations at various levels and approaches based on density functional theory (DFT) which, in the last decade, emerged as a reliable and economic tool for modeling ground state properties and reaction dynamics of intermediates. 

Tel. 800-424-9737, fax 415-491-8311 (U.S.A.), tel. 41-38-337633 (U.K.) Model building, display, charge density, electrostatic potential, and molecular orbital plots. Stick, sphere, and dot surface display. 2D to 3D conversion. Protein and DNA fragment libraries. MM+, BIO+ (implementations of MM2 and CHARMM, respectively), OPLS, and AMBER molecular mechanics and dynamics. Solvent box. Semiempirical calculations by Extended Hiickel, CNDO, INDO, MINDO/3, MNDO, AMI, and PM3. Originated at Hypercube, Inc. (Dr. N. Ostlund et al.), of Ontario, Canada. Runs under Windows on a 386 or 486 PC and under Motif on a Silicon Graphics workstation. 

Ab initio (Hartree-Fock, Moller-Plesset, direct HF), semiempirical (MNDO, AMI, PM3), and molecular mechanics. Graphical front-end and post processor of the output. Cray, Convex, DEC, HP, IBM, and Silicon Graphics versions. 

Semiempirical quantum chemical calculations (MNDO, AMI and PM3) have been performed for determining the possible positions of exchangeable cations and geometries of sorption complexes of cyclopropane and propene in zeolite A. 

Calculations of the barriers for concerted and stepwise transfer of hydrogen to cyclopropene have been carried out using MNDO, AMI and semiempirical SCF-MO methods. ... 

All semiempirical methods are parameterized for the elements H, C, N, F, and O which form the basis of most organic molecules that are of importance in pharmaceutics, biochemistry, and organic chemistry. Here it is possible to compare the statistical errors for some of the more recent methods. In Table 1, the statistics for energetic, structural, and electronic properties for first-row elements are compared for MSINDO, MNDO, AMI, and PM3. 

This review of semiempirical quantum-chemical methods outlines their development over the past 40 years. After a survey of the established methods such as MNDO, AMI, and PM3, recent methodological advances are described including the development of improved semiempirical models, new general-purpose and special-purpose parametriza-tions, and linear scaling approaches. Selected recent applications are presented covering examples from biochemistry, medicinal chemistry, and nanochemistry as well as direct reaction dynamics and electronically excited states. The concluding remarks address the current and future role of semiempirical methods in computational chemistry. 

Sustmann et al. (1993) applied both semiempirical MNDO-AMl and -PM3 calculations and ab initio RHF and CASSCF calculations on different levels (3-21G 6-31G ). Most interesting are the results on the cyclization mechanism with thioformaldehyde (R = H) The ab initio techniques suggest that both regioisomers should be formed in a concerted way, but with MNDO-AMl and -PM3 a concerted cyclization is predicted only for the 1,3,4-thiadiazoline 6.35. The 1,2,3-thiadiazoline 6.36 should be formed via a planar intermediate of type 6.37 in which the distance between the two C-atoms is calculated to be 327 pm (PM3) or 366 pm (AMI). ... 

Because of the difficulties involved in ab initio calculations of potential-energy surfaces for reactions, it would be highly desirable to have a semiempirical method that gives reliable reaction PES results. The MNDO, AMI, and PM3 methods have been widely applied to calculate relevant portions of PESs for chemical reactions (often with inclusion of solvent effects) so as to elucidate reaction mechanisms and transition-state structures. 

Semiempirical methods, for example, MNDO. AMI, and PM3. are simplifications of ab initio molecular orbital theory and employ empirically determined parameters in essence, they only differ in the approximations being made. These methods involve adjustable parameters associated with molecular properties that are calibrated against experimental data. The chief advantage of semiempirical calculations over ab initio calculations is that they are several orders of magnitude faster. Thus, calculations for systems of up to c. 200 atoms are currently possible, whereby with ab initio methods, the limit is a moderately sized molecule (about 50 atoms at the time of writing), if rational results are to be obtained. Frequently, semieinpirical methods have proved to be the computational procedures of choice for studying relatively large molecules. 

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