Additive models, intermolecular interactions interaction potential parameters

Here Eh and a are the parameters of a normal Morse potential, and m = mh mi in the framework of the model discussed). Although this combined potential (4.3.26) provides plausible estimates, it can hardly be substantiated in terms of the theory of intermolecular interactions and contains, in addition, only biquadratic anharmonic... 

The comparison between calculated spectra of the dimer and of the isolated species showed a behaviour which compares favourably with the experimental data. This behaviour can be correlated with the predicted changes of the intramolecular geometry, and in particular with a displacement of the equilibrium BLA parameter, modulated by dipole-dipole intermolecular interaction. Moreover, a simple model based on the explicit introduction of an additional intermolecular dipole-dipole interaction term in the potential allowed to understand the frequency shifts caused by the formation of the dimer. 

In the development of the set of intermolecular potentials for the nitramine crystals Sorescu, Rice, and Thompson [112-115] have considered as the starting point the general principles of atom-atom potentials, proven to be successful in modeling a large number of organic crystals [120,123]. Particularly, it was assumed that intermolecular interactions can be separated into dispersive-repulsive interactions of van der Waals and electrostatic interactions. An additional simplification has been made by assuming that the intermolecular interactions depend only on the interatomic distances and that the same type of van der Waals potential parameters can be used for the same type of atoms, independent of their valence state. The non-electric interactions between molecules have been represented by Buckingham exp-6 functions,... 

The reason that atom-atom potentials are so popular, especially in the study of condensed phases [34] and more complex Van der Waals molecules [35], is that they contain few parameters and can be cheaply calculated, while they still describe (implicitly) the anisotropy of the intermolecular potential and they even model its dependence on the internal molecular coordinates. Moreover, they are often believed to be transferable, which implies that the same atom-atom interaction parameters in Eq. (6) can be used for the same types of atoms in different molecules. One should realize, however, that the accuracy of atom-atom potentials is limited by Eq. (5). Further inaccuracies are introduced when the atom-atom interaction parameters in Eq. (6) are transferred from one molecular environment to another. Furthermore, Eq. (6) does not include a term which represents the induction interactions and there is the intrinsic problem that these interactions are inherently not pairwise additive (see Sect. 1.4). Numerical experimentation on the C2H4-C2H4 and N2-N2 potentials, for example, has taught us [31, 33] that even when sufficient ab initio data are available, so that the terms in Eq. (6) can be fitted individually to the corresponding ab initio contributions and, moreover, the positions of the force centers for each term can be optimized, the average error in the best fit of each contribution still remains about 10%. Since the different contributions to the potential partly cancel each other... 

In real systems, nonrandom mixing effects, potentially caused by local polymer architecture and interchain forces, can have profound consequences on how intermolecular attractive potentials influence miscibility. Such nonideal effects can lead to large corrections, of both excess entropic and enthalpic origin, to the mean-field Flory-Huggins theory. As discussed in Section IV, for flexible chain blends of prime experimental interest the excess entropic contribution seems very small. Thus, attractive interactions, or enthalpy of mixing effects, are expected to often play a dominant role in determining blend miscibility. In this section we examine these enthalpic effects within the context of thermodynamic pertubation theory for atomistic, semiflexible, and Gaussian thread models. In addition, the validity of a Hildebrand-like molecular solubility parameter approach based on pure component properties is examined. 

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