Molecules short-range hard cores

Table 6.1 Contributions of the Keesom, Debye, and London potential energy to the total van der Waals interaction between similar molecules as calculated with Eqs. (6.6), (6.8), and (6.9) using Ctotal = Corient + Cind + Cdisp- They are given in units of 10-79 Jm6. For comparison, the van der Waals coefficient Cexp as derived from the van der Waals equation of state for a gas (P + a/V fj (Vm — b) = RT is tabulated. From the experimentally determined constants a and b the van der Waals coefficient can be calculated with Cexp = 9ab/ (47T21V ) [109] assuming that at very short range the molecules behave like hard core particles. Dipole moments /u, polarizabilities a, and the ionization energies ho of isolated molecules are also listed.

The 12,4 hard-core potential model takes into account repulsive (power of 12 dependence on the distance of approach) and attractive (power of 4 dependence) potentials that arise when the ion and nentral molecule approach each other at short ranges (see Equation 10.20). When a short-range repulsive term is added to the polarization Umit potential of Equation 10.18, the interaction potential is modified to a (n,4)... 

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