United atom: molecular mechanics

As outlined below, molecular mechanics and semiempirical computations directly report heats of formation, a convenience that is not directly available in ab initio computations, which give absolute electronic energies (in atomic units, au). These have little meaning to... 

The existence of a critical thickness of any growing layer can easily be understood without complicated mathematical equations from a schematic diagram shown in Fig. 1.6. This diagram explains the mechanism of physical-chemical processes taking place in the A-AB-B system. Only one of the diffusion paths, along which the B atoms can move (see Fig. 1.4), is shown in Fig. 1.6. To simplify the diagram, the number of molecular AB units was halved in comparison with their number in Fig. 1.4. 

To include the effect of solvent polarization in molecular mechanics, Still and co-workers turned to the generalized Born modelT i i", 194-203 this model, the electric polarization free energy is written in atomic units as... 

There are two reasons why so much is unknown. First, at high densities three (and even four) body forces are important. This is particularly so when chemically reactive atoms are present. Then, even for two-body forces, the strongly repulsive regime is not well understood and, in addition, close in, as one approaches the united atom limit, there is considerable promotion of molecular orbitals. This is a universal mechanism for electronic excitation which means a breakdown of the Born-Oppenheimer approximation for close collisions. 

In order to minimize effects of nanotube edges in our molecular mechanics calculations we have chosen five unit cells (480 carbon atoms total) of (8,8) CNT. The calculated dependence of the interaction energy between the fullerene C20 and the (8,8) CNT on the fullerene displacement along the CNT axis is shown in Fig. 3. The calculated period of this dependence is a half of the translational period of the Kekule structure (Fig. 1). The small difference between two barriers in Fig. 3 is due to the edge effects. Correspondingly, the frequency of small oscillation of fullerene C20 along the CNT axis near the minimum of potential energy is v 60 GHz. 

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