Quantum-chemical cluster models

Fig. 6.87. Density of electronic states of copper (111) surface calculated from quantum chemical modeling for the copper surface alone and with the water molecules adsorbed on top. The solid line represents the cluster (a) Cu19 and (b) Cu 9, and the dashed lines (a) Cu19(H20)5 and (b) Cu19(H20)5. (Reprinted from R. R. Nazmut-dinov and M. S. Shapnik, Electrochem. Acta 41 2253, copyright 1996, Fig. 4, with permission from Elsevier Science.)...

For very small metal-oxide clusters, the band-gap increases. This becomes a concern for clusters which are just a few nanometer in diameter. This effect is most readily understood in the quantum chemical model of a cluster, where the bands develop due to interactions between more and more atoms. For clusters with, say, up to 1000 atoms, the addition of new atoms to the cluster broadens the bands, and thus narrows the band-gaps. For large clusters this effect levels off, and for clusters with a diameter of more than a few nanometers band-widths and band-gaps stabilize at their bulk values. 

The parameters of Hamiltonians (1) and (2) are determined in our approach by pure theoretical way using different quantum chemical models and calculations unlike the traditional fitting the experimental thermodynamic and dielectric data. Our method of the many-pseudospin clusters [ 1,4] seems to be the most reliable way of determination. The latter are obtained in this case within the static approximation from the system of equations for a typical crystal fragment (cluster) for all possible proton distributions on H-bonds. The left-hand side of any equation expresses the cluster total energy in terms of Jy, while the right-hand side is determined by means of the quantum chemical calculation of this energy. 

Thus, comparison of calculation results with experimental data demonstrates ab initio Hartree-Fock method in the 6-3 IG basis set provides qualitatively correct conclusion on hydrogen bond energies for systems considered. This model was applied to investigate the interaction between water and hydroxyfullerene cluster Qo. Hydroxyfullerene is a proper object for quantum-chemical modeling of interaction between activated carbon nanoparticle and water. At the same time hydroxyfullerene is of great interest because of its possible applications in medicine, for water disinfection, and for polishing nanosurfaces. 

Keywords Silica Clusters / Meta-Silicon Acid / Quantum-Chemical Modeling / Semiempirical AMI and PM3 Approximations / Vibrational Spectroscopy... 

The influence of nanostmctuies on the media and compositions was discussed based on quantum-chemical modeling [9]. After comparing the energies of interaction of fullerene derivatives with water clusters, it was found that the increase in the interactions in water medium under the nanostmcture influence is achieved only with the participation of hydroxyfullerene in the... 

It is thus obvious that among numerous computational methods, first principles quantum chemical approach is indispensable. However, initially first principles quantum chemical calculations required the use of models consisting of a few atoms (clusters) and the range of properties was limited. Since the advent of modem computing resources, as well the models could be extended to cover larger variety of structures as the methodology has been... 

Quantum mechanics is essential for studying enzymatic processes [1-3]. Depending on the specific problem of interest, there are different requirements on the level of theory used and the scale of treatment involved. This ranges from the simplest cluster representation of the active site, modeled by the most accurate quantum chemical methods, to a hybrid description of the biomacromolecular catalyst by quantum mechanics and molecular mechanics (QM/MM) [1], to the full treatment of the entire enzyme-solvent system by a fully quantum-mechanical force field [4-8], In addition, the time-evolution of the macromolecular system can be modeled purely by classical mechanics in molecular dynamicssimulations, whereas the explicit incorporation... 

A specialized MOPAC computer software package and, in particular, its PM3 quantum-chemical program has been successfully applied in calculations. The results of calculations have shown that both oxygen atoms form bonds with two more active carbon atoms of CP molecular cluster (so-called bridge model of adsorption). The total energy of system after a chemical adsorption at such active atoms is minimal. 

The requirements for Raman resonance that must be fulfilled are the following (120,121) (a) total symmetry of the vibrations with respect to the absorbing center, and (b) same molecular deformation induced by the electronic and vibrational excitations. Quantum chemical calculations (41) of the vibrational frequencies and the electronic structure of shell-3 cluster models allowed the assignment of the main vibrational features, as shown in Fig. 7. The 1125 cm-1 band is unequivocally assigned to the symmetric stretching of the Ti04 tetrahedron. 

Beyond the clusters, to microscopically model a reaction in solution, we need to include a very big number of solvent molecules in the system to represent the bulk. The problem stems from the fact that it is computationally impossible, with our current capabilities, to locate the transition state structure of the reaction on the complete quantum mechanical potential energy hypersurface, if all the degrees of freedom are explicitly included. Moreover, the effect of thermal statistical averaging should be incorporated. Then, classical mechanical computer simulation techniques (Monte Carlo or Molecular Dynamics) appear to be the most suitable procedures to attack the above problems. In short, and applied to the computer simulation of chemical reactions in solution, the Monte Carlo [18-21] technique is a numerical method in the frame of the classical Statistical Mechanics, which allows to generate a set of system configurations... 

Simple quantum chemical upper and lower bounds for energies of cluster models... 

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