Intermolecular forces, parametrization

Thornton, B. H. and Bogy, D. B., "A Parametric Study of Head-Disk Interface Instability Due to Intermolecular Forces, IEEE Trans. Magn., Vol. 40, No. (1), 2004, pp. 337-343. 

Inter- and intramolecular forces (imf) are of vital importance in the quantitative description of structural effects on bioactivities and chemical properties. They can make a significant contribution to chemical reactivities and some physical properties as well. Types of intermolecular forces and their present parametrization are listed in Table 7. 

The chemist is concerned with the relative positions of the nuclei in the molecule and with the internal energy, but not with the motions of the molecule as a whole. This motion can be excluded from our consideration, for example, by elimination of the center-of-mass translation. Moreover, the intermolecular (electrostatic) forces acting on electrons and nuclei would be similar. This would cause much slower internal motion of the heavy nuclei in comparison to light electrons. For this reason, the approximate description of electron motion with parametric dependence on the static positions of nudei is justifled. Such reasoning leads to adiabatic approximation and Anally to Born-Oppenheimer approximation. 

The philosophical transition from the atomic prejudice to a view of intermolecular interaction in terms of diffuse electron density has its proper computational counterpart in full quantum mechanical calculations, which, however, cannot at present provide complete intermolecular energies because of limitations in the treatment of electron correlation, a major ingredient of the intermolecular interaction recipe. In a different perspective, the classical atom-atom force-field approach is widely applicable but entirely parametric and of scarce adherence to physical principles. The need is felt for an extension to represent in a more realistic manner the effects of diffuse electron clouds. This is done in the so-called semi-classical density sums (SCDS) or briefly. Pixel approach [9], which will now be described. The Pixel method is based on numerical integrations over molecular electron densities, and allows a separation of the total intermolecular cohesion energy into coulombic, polarization, dispersion, and repulsion contributions. 

Parametrization is made by a best fit to either empirical data or data obtained by quantum mechanical calculations. Since all parameters are adjusted at the same time and since the same parameters are used for similar bonding situations in different systems, the parameters obtained do not have the physical meaning that the functional form chosen may imply. For example, the parameters used for the nonbond term in a force field may differ from the parameters of the same expression used for describing intermolecular interactions. The reason is that in the former case part of the atom-atom... 

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