Computer modelling, carbon structures

To evaluate further the CAMD results, a number of atomic and chemical parameters from each structure (number of atoms, fractions of aromatic carbon and hydrogen, weight fraction or each atomic species, empirical formula) were compared with the original literature for each structure. This provided a useful check on the accuracy of the computer models. Results of the computer analyses for the four coal structures are given in Table I. The total numbers of atoms only appear as guides to the size and complexity of each structure, and bear no relationship to the size of a "coal molecule" or a decomposition product. 

Fig. 4. Computer-generated crystal structure models nop row. left to right) Cuprite, zinc-blende, rutile, perovskite. iridymite (second row) Cristobalite. potassium dihydrogen phosphate, diamond, pyrites, arsenic (third rowt Cesium chloride, sodium chloride, wurtzite. copper, niccolite (fourth row) Spinel, graphite, beryllium, carbon dioxide, alpha i uanz. [AT T Bel Laboratories ...

Because they are so computationally intensive, ab initio and semiempirical studies are limited to models that are about 10 rings or less. In order to study more reahstic carbon structures, approximations in the form of the Hamiltonian (i.e., Schrodinger equation) are necessary. The tight-binding method, in which the many-body wave function is expressed as a product of individual atomic orbitals, localized on the atomic centers, is one such approximation that has been successfully applied to amorphous and porous carbon systems [47]. 

Lastly, the single-events theory, which was historically designed for the activation of carbon-carbon bonds, does not currently cover the reactivity of C-S and C-N bonds. Some computer models have been produced to represent the possible presence of hetero atoms in hydrocarbon structures. Avenues are therefore open for a very wide field of application, that of HDT reactions and sulphide catalysis. They must nevertheless be based on an in-depth, improved description of the heterolytic mechanisms, also studied (Blanchin et al., 2001) under IFP supervision. 

Most of the work done to date is based on classical effective potentials that describe with sufficient accuracy the relevant interatomic and intermolecular forces, in particular the vdW couplings between hydrogen molecules and the various carbon structures considered (mainly graphite and nanotubes). An important point to note is that, since the electrons are not explicitly treated in such models, the computational cost of the simulations is relatively small, which makes it possible to perform, for realistic model systems composed of hundreds of atoms, sophisticated statistical calculations in the grand-canonical ensemble, and thus compute the amount of hydrogen that can be stored at particular temperature and pressure conditions, and even consider the quantum effects associated to the hydrogen molecules. Useful reviews of the most important results and literature can be found in Hirscher and Becher, Meregalh and Parrinello and Simonyan and Johnson. ... 

We take advantage of a simple computational model [13,27-31] exploiting the C carbon coordinates of the kinesin homodimer in its native state. For simplicity, we introduce Go-like interactions between the residues, so that the native structure takes the minimum energy conformation. Our study based on this simple model finds that... 

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