Computer programs bond valence

Electrostatic stabilization, 181, 195,225-228 Empirical valence bond model, see Valence bond model, empirical Energy minimization methods, 114-117 computer programs for, 128-132 convergence of, 115 local vr. overall minima, 116-117 use in protein structure determination,... 

Quantum-chemical cluster models, 34 131-202 computer programs, 34 134 methods, 34 135-138 for chemisorption, 34 135 the local approach, 34 132 molecular orbital methods, 34 135 for surface structures, 34 135 valence bond method, 34 135 Quantum chemistry, heat of chemisorption determination, 37 151-154 Quantum conversion, in chloroplasts, 14 1 Quantum mechanical simulations bond activation, 42 2, 84—107 Quasi-elastic neutron scattering benzene... 

Exclusion Principle. The orbitals are calculated in a self-consistent fashion in a manner analogous to those developed previously for atomic orbitals and are based on linear combination of the atomic orbitals of the individual atoms. The number of molecular orbitals equals the number of atomic orbitals in the atoms being combined to form the molecule. A molecular orbital describes the behavior of one electron in the electric field generated by the nuclei and some average distribution of the other electrons. This approximation proved to be more amenable to computer programming than the valence bond model and was widely developed and used in increasingly less approximate forms from 1960 to 1990. 

The first quantitative theory of chemical bonding was developed for the hydrogen molecule by Heitler and London in 1927, and was based on the Lewis theory of valence in which two atoms shared electrons in such a way that each achieved a noble gas structure. The theory was later extended to other, more complex molecules, and became known as valence bond theory. In this approach, the overlap of atomic orbitals on neighbouring atoms is considered to lead to the formation of localized bonds, each of which can accommodate two electrons with paired spins. The theory has been responsible for introducing such important concepts as hybridization and resonance into the theory of the chemical bond, but applications of the theory have been limited by difficulties in generating computer programs that can deal efficiently with anything other than the simplest of molecules. 

The early popularity of valence bond methods declined, because the molecular orbital calculations were more amenable to writing efficient computer algorithms. Recently the programming of valence bond methods has improved because it also benefitted from the rapid development of computer technology and programs have been developed which are competitive in accuracy and economy with programs for the Hartree-Fock method and other molecular orbital-based methods. These developments are due to and have been described by Gerratt, Cooper, Karadakov and Raimondi and summarised by Li and McWeeny in 2002, van Lenthe and co-workers in 2005 and Shaik and Hiberty s reviews in 2008 [155-158]. 

Computer programs have been developed which can transform the results of molecular orbital calculations into NBOs. An optimal Lewis structure can be defined as that one with the maximum amount of electronic charge in Lewis orbitals (Lewis charge). A low amount of electronic charge in Lewis orbitals indicates strong effects of electron delocalisation [226-228]. In resonance structures, major and minor contributing structures may exist. These analyses provide results which are similar to modem valence bond theory methods. 

Chemical structures are typically represented in computer programs as bidirectional graphs used to depict a valence-bond theory model of the chemical structure. Atoms are represented by a list of nodes and bonds by a list of edges. Typically there are some constraints imposed for chemical sense (e.g., valence control), and only predefined properties can be associated with each (e.g., formal charge, bond order). Other data can be associated with the full structure (e.g., chirality, molecular formula, or any other data stored in database fields) (see Figure 1). 

Draw bond order and free valency index diagrams for the butadienyl system. Write a counter into program MOBAS to detemiine how many iterations are executed in solving for the allyl system. The number is not the same for all computers or operating systems. Change the convergence criterion (statement 300) to several different values and determine the number of iterations for each. 

Several methods of quantitative description of molecular structure based on the concepts of valence bond theory have been developed. These methods employ orbitals similar to localized valence bond orbitals, but permitting modest delocalization. These orbitals allow many fewer structures to be considered and remove the need for incorporating many ionic structures, in agreement with chemical intuition. To date, these methods have not been as widely applied in organic chemistry as MO calculations. They have, however, been successfully applied to fundamental structural issues. For example, successful quantitative treatments of the structure and energy of benzene and its heterocyclic analogs have been developed. It remains to be seen whether computations based on DFT and modem valence bond theory will come to rival the widely used MO programs in analysis and interpretation of stmcture and reactivity. 

The PPP-MO method is suitable for the treatment of large molecules, does not present major computing demands and programs are now routinely used as a tool to calculate the colour properties of dyes. Unlike the HMO method, it handles heteroatomic species well. The method has been remarkably successful in calculating /lmax values for a wide range of dyes from virtually all of the chemical classes. For example, the method provides a reasonably accurate account of substituent effects in the range of aminoazobenzene dyes, including compounds 15a-f and 16a-f which have been discussed in terms of the valence-bond approach in the previous section of this chapter. 

With the advent of the stored-program digital computer a minor revolution occurred in quantum chemistry. The integrals appearing in the models being used for small molecules were actually evaluated and it became clear that molecules were enormously more complicated than had been anticipated. The oversimplified valence bond and molecular orbital methods often gave qualitatively ridiculous results when taken literally (11). 

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