Potential energy surfaces intermolecular forces

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 one-dimensional potential curves depicted in Figures 1.1-1.3 represent the dissociation of diatomic molecules for which the potential V(Rab) depends only on the internuclear distance between atoms A and B. However, if one or both constituents are molecules, V is a multidimensional object, a so-called potential energy surface which depends on several (at least three) nuclear coordinates denoted by the vector Q = (Ql,Q2,Q3,---) (Margenau and Kestner 1969 Balint-Kurti 1974 Kuntz 1976 Schaefer III 1979 Kuntz 1979 Truhlar 1981 Salem 1982 Murrell et al. 1984 Hirst 1985 Levine and Bernstein 1987 ch.4 Hirst 1990 ch.3). The intramolecular and intermolecular forces, defined by... 

Basis set superposition error (BSSE) is a particular problem for supermolecule treatments of intermolecular forces. As two moieties with incomplete basis sets are brought together, there is an unavoidable improvement in the overall quality of the supermolecule basis set, and thus an artificial energy lowering. Various approximate corrections to BSSE are available, with the most widely used being those based on the counterpoise method (CP) proposed by Boys and Bemardi [3]. There are indications that potential energy surfaces corrected via the CP method may not describe correctly the anisotropy of the molecular interactions, and there have been some suggestions of a bias in the description of the electrostatic properties of the monomers (secondary basis set superposition errors). 

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. 

One of the growth areas in theoretical chemistry has been the attention paid to the potential energy surfaces for molecules and interacting molecules, rather than just equilibrium properties. Bader and Gangi provide a very detailed and comprehensive review of ab initio calculations on potential energy surfaces, and Stamper deals with the subject of intermolecular forces. 

Potential energy surfaces of weakly bound dimers and trimers are the key quantities needed to compute transition frequencies in the high resolution spectra, (differential and integral) scattering cross sections or rate coefficients describing collisional processes between the molecules, or some thermodynamic properties needed to derive equations of state for condensed phases. However, some other quantities governed by weak intermolecular forces are needed to describe intensities in the spectra or, more generally, infrared and Raman spectra of unbound (collisional complexes) of two molecules, and dielectric and refractive properties of condensed phases. These are the interaction-induced (or collision-induced) dipole moments and polarizabilities. 

The Ar-HCl and Ar-HF complexes became prototypes for the study of intermolecular forces. Holmgren et al [30] produced an empirical potential energy surface for Ar-HCl fitted to the microwave and radiofrequency spectra,... 

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