Intermolecular Forces and Potential Energies

The presence of forces among the molecules of a system gives rise to their potential energy which, along with their kinetic energy, comprise the internal energy of the system. The potential energy F between two molecules, whose centers are at a distance r, is related to the intermolecular force F by  

Equation 7.3.1 is applicable when the potential energy is a function of the distance between the centers of the molecules only, which is the case for small symmetric molecules (Ar, Xe, CH4, etc.). 

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As shown above, various types of molecules exhibit different intermolecular forces, and their different force and potential-energy functions can be estimated. If the potential-energy function were known for all the atoms or molecules in a system, as well as the spatial distribution of all... 

We are treating interactions in terms of energies, but they can also be treated as forces, because all contributions to intermolecular interactions studied in Section 4.3.3 cause forces to act on the molecules the relation between force and potential energy is well known ... 

The problem of influence of the electric field intensity on the permittivity of solvents has been discussed in many papers. The high permittivity of water results from the intermolecular forces and is a cumulative property. The electric field intensity is the lowest at the potential of zero charge (pzc), thus allowing water molecules to adsorb in clusters. When the electrode is polarized, the associated molecules, linked with hydrogen bonds, can dissociate due to a change in the energy of their interaction with the electrode. Moreover, the orientation of water molecules may also change when the potential is switched from one side of the pzc to the otha. 

It is beyond the scope of this review to cover in depth either valence theory or the theory of intermolecular forces and I shall only attempt to deal with some general principles of both which appear to be important for an understanding of potential energy surfaces. Before dealing separately with weak and strong interactions, there is one point they have in common and that is the increasing computational effect that is required as the number of internal coordinates increases. 

The ab initio spin-coupled valence bond (SCVB) approach continues to provide accurate ground and excited state potential energy surfaces for use in a variety of subsequent applications, with particular emphasis on intermolecular forces and reactive systems. The compactness of the various wavefunctions allows direct and clear interpretation of the correlated electronic structure of molecular systems. Recent developments, in the form of SCVB and MR-SCVB, involve the optimization of virtual orbitals via an approximate energy expression. These improved virtuals lead to still higher accuracy for the final variational wavefunctions, but with even more compact wavefunctions. 

Introduction.—Most theoretical v> ork on the relation between intermolecular forces and the thermodynamic properties of liquids and liquid mixtures has been limited to potential fields which are independent of the orientation of the particles. This condition, however, is only strictly satisfied by monoatomic substances. For a great many molecular substances directional intermolecular forces are likely to be important and will have a significant effect on the thermodynamic properties of liquids both because of the additional cohesive energy and because of the loss of entropy associated with hindrance to free rotation. Although many of the observed properties of liquids have been attributed to directional forces in a qualitative manner, there has been little in the way of general quantitative theory. 

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