Organic molecules semiempirical molecular orbital

P.L. Cununins and J.E. Gready, Coupled semiempirical molecular orbital and molecular mechanics model (QM/MM) for organic molecules in aqueous solution, J. Comput. Chem., 18(1997), 1496-1512. 

SattehneyCT KW, Tirado-Rives J, Jorgensen WL (2006) Comparison of SCC-DFTB and NDDO-based semiempirical molecular orbital methods for organic molecules. J Phys Chem A 110 13551-13559... 

The KT approximation within the ab initio and semiempirical molecular orbital formalisms is unsatisfactory even for simple and well-studied organic molecules such as benzene and pyridine, as can be seen from the results presented in Table 1. 

Hie canonical MOs of a planar unsaturated organic molecule can be divided into molecular plane is +1 or -1, respectively (Section 15.10). The earliest semiempirical methods for planar conjugated organic compounds (Sections 16.2-16.4) treated the tt electrons separately from the cr electrons. Coulson stated that the justification for the ct-tt separability approximation lies in the different symmetry of the cr and tt orbitals and in the greater polarizability of the tt electrons, which makes them more susceptible to perturbations such as those occurring in chemical reactions. 

At the same time, molecular orbitals were used in simplified treatments of the pi-electrons in conjugated organic molecules. Similar empirical or semiempirical electron-structure models were developed and successfully applied in other areas of chemistry. They have been described in other chapters of this book. The field was therefore open for development of a nonempirical HF theory for molecules. A key paper was published by Roothaan in 1951. Albeit not the first to do so, he gave a clear and detailed description of an HF procedure for molecules that could almost be used as a manual for the development of a computer code. The computers were not quite ready, though, and it was to take another 10 years before the first general-purpose codes were produced. 

Tire calculations of the molecular orbital eneigies c (which we employ in the same manner as is usual in the semiempirical methods for organic molecules) and the evaluation of the coefficients of the atomic orbitals and sets of atomic orbitals in Table I in the final molecular orbital requires the solution of secular determinants (one for each irreducible representation) of the form C iy4[ =0, where Ha has its usual... 

Molecular orbitals obtained from ab initio or semiempirical calculations can also be represented by isovalue surfaces (Figures 10 and 11). Visualized HOMOs (highest occupied molecular orbital) and LUMOs (lowest unoccupied molecular orbital) are often used to display reactive pathways of organic molecules. 

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