Computational chemistry semi-empirical methods

Stewart J J P 1989a. Optimisation of Parameters for Semi-empirical Methods 1. Method. Journal of Computational Chemistry 10 209-220. 

J. Sadley, Semi-Empirical methods of Quantum Chemistry, Wiley, 1985 M. C. Zemer, Rev. Comput. Chem., 2 (1991), 313. 

An overview is presented of the state-of-the-art for quantum chemical calculations for d- and f- electron systems. The present role and the potential of ab-initio, density functional and semi-empirical methods are discussed with reference to contemporary developments in related e35>erimental disciplines. Progress towards a true computational chemistry including the transition metals, lanthanides, and actinides is outlined with emphasis both on achievements and on the remaining barriers. 

In addition to the chemistry that may be addressed, ab-initio calculations are needed to provide important guidance on which to develop semi-empirical and empirical schemes that use, in fact, the same formalism. There is an important interplay between all methods, and the information that we gain from ab-initio calculations is not only used to examine the chemistry of the model systems, but also used to help develop semi-empirical methods. The numbers obtained are often used to guide parameter choices where these are not available from experiment. Most importantly, discoveries in the underlying theory that we use are made in semi- empirical, density functional and ab-initio studies, and these improvements are adopted, perhaps in modified form, to affect all theories used in computational chemistry. 

The equation is used to describe the behaviour of an atom or molecule in terms of its wave-like (or quantum) nature. By trying to solve the equation the energy levels of the system are calculated. However, the complex nature of multielectron/nuclei systems is simplified using the Born-Oppenheimer approximation. Unfortunately it is not possible to obtain an exact solution of the Schrddinger wave equation except for the simplest case, i.e. hydrogen. Theoretical chemists have therefore established approaches to find approximate solutions to the wave equation. One such approach uses the Hartree-Fock self-consistent field method, although other approaches are possible. Two important classes of calculation are based on ab initio or semi-empirical methods. Ah initio literally means from the beginning . The term is used in computational chemistry to describe computations which are not based upon any experimental data, but based purely on theoretical principles. This is not to say that this approach has no scientific basis - indeed the approach uses mathematical approximations to simplify, for example, a differential equation. In contrast, semi-empirical methods utilize some experimental data to simplify the calculations. As a consequence semi-empirical methods are more rapid than ab initio. 

Smith G D and R L Jaffe 1996. Quantum Chemistry Study of Conformational Energies and Rotational Energy Barriers in n-Alkanes. Journal of Physical Chemistry 100.18718-18724 Stewart J J P 1989a. Optimisation of Parameters for Semi-empirical Methods I. Mefliod. Journal of Computational Chemistry 10 209-220... 

Bezier B H, K M Merz Jr and P A Kollman 1990. Atomic Charges Derived from Semi-Empirical Methods Journal of Computational Chemistry 11 431-439... 

IR spectra of pure acetylacetone and acetylacetone with silver were calculated with different methods of computational chemistry. PM3 method was shown closest results with respect to experimental data among semi-empirical methods. It was established that there is no strong dependence between size of ab initio method basis set and data accuracy. It was found that IR spectrum carried out with ab initio method with small basis set 6-2IG has more accurate data than analogous one with laige basis set 6-3 IG. Experimental and calculated spectra were shown identical picture of spectral changes with except of imavoidable calculating error. 

AMI Austin Mcxiel 1, a semi-empirical method for the quantum calculation of molecular electronic structure in computational chemistry based on NDDO." ... 

Marathon Oil developed a thermochemical process based on the oxidizing ability of i-butylanthraquinone [14] and further studied by Mark et al. [15] who employed computational chemistry using semi-empirical methods to determine the detailed chemical steps of the process occurring through Sg formation. 

For TT-electron systems, this was the Hiickel method proposed by Erich Hiickel. For all valence electron systems, the extended Hiickel method was proposed by Roald Hoffmann. Semi-empirical calculations are much faster than their ab initio coxmter-parts. Their results, however, can be very wrong if the molecule being computed is not similar enough to the molecules in the database used to parametrize the method. Semi-empirical calculations have been most successful in the description of organic chemistry, where only a few elements are used extensively and molecules are of moderate size. However, semi-empirical methods were also appHed to solids and nanostructures but with different parameterization. As with empirical methods, we can distinguish if ... 

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