Intermolecular forces square well potential

The rectangular form of the well, being cmde, nevertheless allows us to obtain a rather adequate description of local-order intermolecular forces arising in a liquid, which generally presents a state intermediate between solid body and gas. We emphasize that popular [13, 14] parabolic, cosine, or cosine-squared potential wells generally give poor description of the wideband... 

Fig. 2-1 Models of intermolecular potentials, (a) Forceless mass points (b) elastic hard spheres (c) elastic hard spheres with superposed central attractive forces (d) molecules with central finite repulsive and attractive forces (e) square-well model (f) point centers of inverse-power repulsion or attraction.

One model potential used for intermolecular forces combines the square well and an exponential... 

The second simulation technique is molecular dynamics. In this technique, which was pioneered by Alder, initial positions of theparticles of a system of several hundred particles are assigned in some way. Displacements of the particles are determined by numerically simulating the classical equations of motion. Periodic boundary conditions are applied as in the Monte Carlo method. The first molecular dynamics calculations were done on systems of hard spheres, but the method has been applied to monatomic systems having intermolecular forces represented by the square-well and Lennard-Jones potential energy functions, as well as on model systems representing molecular substances. Commercial software is now available to carry out molecular dynamics simulations on desktop computers. ... 

The H-bond stretching frequency provides a measure of the curvature of the bottom of the well in the intermolecular potential. It is therefore instructive to point out that correlates fairly well with the interaction energy. These frequencies are rather small, on the order of 100 cm", and occur in the far-IR region of the spectrum. As a point of comparison, the stretching frequency of the C—F covalent bond typically appears at approximately 1000 cm". Since the force constant is proportional to the square of the frequency, one can say that the curvature of the H-bond potential is only some 1/100 that of the C —F bond. 

---