Nonempirical molecular orbital method

A. Wallqvist and G. Karlstrbm, Chem. Scripta, 29A, 131 (1989). A New Nonempirical Force Field for Computer Simulations. (This NEMO method of this paper should not be confused with the earlier molecular orbital method with the same name see M. D. Newton, F. P. Boer, and W. N. Lipscomb, /. Am. Chem. Soc, 88, 2353 (1966). Molecular Orbitals for Large Molecules.) C. E. Dykstra, Chem. Rev., 93,2339 (1993). Electrostatic Interaction Potentials in Molecular Force Fields. 

Molecular orbital calculations have been performed on compounds 19 and 20 . The calculated PM3 equilibrium geometric structures show that these compounds are severely distorted from planarity in accordance with X-ray structural analysis (see Section 8.I2.3.I). On the other hand, PM3 calculations performed on both neutral and oxidized/reduced compounds show that oxidation and reduction induce a clear gain of aromaticity. Predictions using the nonempirical valence effective Hamiltonian (VEH) method have shown that the electronic charge density in the highest occupied molecular orbital (HOMO) is localized on the benzodithiin 19 or benzoxathiin 20 rings. 

The ground-state wave function of cytosine has been calculated by practically all the semiempirical as well as nonempirical methods. Here, we shall discuss the application of these methods to interpret the experimental quantities that can. be calculated from the molecular orbitals of cytosines and are related to the distribution of electron densities in the molecules. The simplest v-HMO method yielded a great mass of useful information concerning the structure and the properties of biological molecules including cytosines. The reader is referred to the book1 Quantum Biochemistry for the application of this method to interpret the physicochemical properties of biomolecules. Here we will restrict our attention to the results of the v-SCF MO and the all-valence or all-electron treatments of cytosines. 

Th6 theory and the analysis of calculational schemes of quantum chemistry have been dealt with in detail in a number of books [1-6] and review articles [7-12]. Here we give only a brief account of the main principles of the general theory of molecular orbitals (MO) that provides the basis for constructing the most important nonempirical methods of quantum chemistry. 

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