Methane force helds

Molecular solids have their lattices composed of molecules held in place by London forces, dipole-dipole forces, and hydrogen bonding. Solid methane and water are example of molecular solids. 

Bandyopadhyay and Yashonath (31), in an extension of their work on MD studies of noble gas diffusion, presented MD results for methane diffusion in NaY and NaCaA zeolites. The zeolite models were the same as those used in the noble gas simulations (13, 15, 17, 18, 20, 28, 29) and the zeolite lattice was held rigid. The methane molecule was approximated as a single interaction center and the guest-host potential parameters were calculated from data of Bezus et al. (49) (for the dispersive term) and by setting the force on a pair of atoms equal to zero at the sum of their van der Waals radii (for the repulsive term). Simulations were run for 600 ps with a time step of 10 fs. 

Neither methane nor carbon tetrachloride is readily soluble in water. The highly polar water molecules are held to each other by very strong dipole-dipole interactions—hydrogen bonds there could be only very weak attractive forces between water molecules on the one hand and the non-polar methane or carbon tetrachloride molecules on the other. 

The structure of ice represents an interesting contrast to that of crystalline methane. See Section 9.12. Methane crystals are held together by weak dispersion forces, which are largely independent of the relative orientation of nearest neighbor molecules. The lattice is close-packed cubic, each CH4 molecule is surrounded by 12 nearest neighbors at C- C distances of 316 pm. Ice crystals, on the other hand, are held together by hydrogen bonds. 

Weaker secondary bonds act between molecules. Thus, below — 182°C, methane is a solid, the covalent molecules being held in a solid lattice be weak secondary bonds. These weak forces are associated with interactions between dipoles. Three different types of interaction have been described by London, Debye and Keesom, known respectively as dispersion, induction and orientation forces see Table 1 and Dispersion forces and Polar forces. The three types of interaction are often referred to collectively as van der Waals forces, as indicated in Table 1. However, it is necessary to note that some authors use the term van der Waals to refer exclusively to dispersion forces, the other two types being referred to as polar forces . Table 2. (The term dispersive is sometimes used by francophone authors writing in English where dispersion would be correct.)... 

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