Semiempirical techniques

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. 

One formalism which has been extensively used with classical trajectory methods to study gas-phase reactions has been the London-Eyring-Polanyi-Sato (LEPS) method . This is a semiempirical technique for generating potential energy surfaces which incorporates two-body interactions into a valence bond scheme. The combination of interactions for diatomic molecules in this formalism results in a many-body potential which displays correct asymptotic behavior, and which contains barriers for reaction. For the case of a diatomic molecule reacting with a surface, the surface is treated as one body of a three-body reaction, and so the two-body terms are composed of two atom-surface interactions and a gas-phase atom-atom potential. The LEPS formalism then introduces adjustable potential energy barriers into molecule-surface reactions. 

Semiempirical techniques are the next level of approximation for computational simulation of molecules. Compared to molecular mechanics, this approach is slow. The formulations of the self-consistent field equations for the molecular orbitals are not rigorous, particularly the various approaches for neglect of integrals for calculation of the elements of the Fock matrix. The emphasis has been on versatility. For the larger molecular systems involved in solvation, the semiempirical implementation of molecular orbital techniques has been used with great success [56,57]. Recent reviews of the semiempirical methods are given by Stewart [58] and by Rivail [59],... 

Ab initio quantum mechanical (QM) calculations represent approximate efforts to solve the Schrodinger equation, which describes the electronic structure of a molecule based on the Born-Oppenheimer approximation (in which the positions of the nuclei are considered fixed). It is typical for most of the calculations to be carried out at the Hartree—Fock self-consistent field (SCF) level. The major assumption behind the Hartree-Fock method is that each electron experiences the average field of all other electrons. Ab initio molecular orbital methods contain few empirical parameters. Introduction of empiricism results in the various semiempirical techniques (MNDO, AMI, PM3, etc.) that are widely used to study the structure and properties of small molecules. 

A variety of computational methods have been compared in the prediction of bond lengths, bond angles, and the heat of formation of thiazole <93JCC75>. Among these methods are the semiempirical techniques AMI, PM3, and MNDO, and the thermochemical basis sets of Benson. 

Assuming that an ab initio or semiempirical technique has been used to obtain p(r), we address the important question of how the calculated electrostatic potential depends on the nature of the wavefunction used for computing p(r). Historically, and today as well, most ab initio calculations of V(r) for reasonably sized molecules have been based on self-consistent-field (SCF) or near Hartree-Fock wavefunctions and therefore do not reflect electron correlation. Whereas the availability of supercomputers has made post-Hartree-Fock calculations of V(r) (which include electron correlation) a realistic possibility even for molecules with 5 to 10 first-row atoms, there is reason to believe that such computational levels are usually not necessary and not warranted. The M0l er-Plesset theorem states that properties computed from Hartree-Fock wavefunctions using one-electron operators, as is V(r), are correct through first order " any errors are no more than second-order effects. Whereas second-order corrections may not always be insignificant, several studies have shown that near-Hartree-Fock electron densities are affected to only a minor extent by the inclusion of correlation.The limited evidence available suggests that the same is true of V(r), ° ° as is indicated also by the following example. 

In order to address these issues, Beckwith and Zavitsas explored the use of the AMI semiempirical technique to model the reactivities and diastereoselectivities during hydrogen transfer from BujSnH to a series of radicals 66 derived from... 

Frequently, different semiempirical techniques are employed simultaneously and the results are compared sometimes they are used by turns to reproduce a particular property. A study of the molecular geometry and electronic structure of condensed pentatomic heterocycles was carried out in this way, optimizing the molecular geometry at MNDO level while the electronic spectra were calculated at different CNDO levels <87JCPB1147>. 

In Pople s career in computational chemistry he was first instrumental in the introduction and dissemination of semiempirical techniques, and contributed to the development of a whole set of successful methods that gained broad acceptance and applications. Here we mention only two of the several major figures in addition to Pople who contributed greatly to the development and spreading of semiempirical methods, Robert G. Parr (Fig. 1.16a) and Michael Dewar (Fig. 1.16b). 

This textbook on the application of ab initio and semiempirical techniques for the analysis of organic reaction mechanisms is designed for chemistry undergraduates. 

A wide variety of heat exchanger units can be used in process work. Very often, complex flow patterns are used to enhance the heat transfer. These patterns make it necessary to use special semiempirical techniques to design such units. 

Much effort is currently devoted to developing more economical quantum-chemical methods for biopolymers. Semiempirical techniques are very inaccurate (even compared to force fields) for description of molecular interactions [12d,e]. More promising are the Density Functional Theory methods [14]. A high quality DFT method can provide, for some applications, results comparable to those obtained by good quality, traditional ab initio techniques. However, high quality DFT techniques are still costly. Furthermore, no DFT method currently works for dispersion-controlled interactions [12b, 15]... 

Quantum mechanical methods have been applied to carbohydrates and their fragments, especially for the purpose of understanding anomeric effects. Semiempirical techniques such as AMI and PM3 have been applied a number of times, even to molecules as large as cyclooctaamylose, which contains 168 atoms. Although these methods are often the only viable alternatives for studying chemical reactivity, they do not enjoy a reputation for accuracy sufficient to answer many questions. Density functional theory and ab initio methods have also been applied, again with geometry optimization. Solvation has also been included in some of these studies, based on continuum methods, which continue to be developed. ... 

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