Anisotropic atom-atom potentials

The forces between polyatomic molecules are traditionally represented by the isotropic atom-atom potential model. However, the implicit assumption that the atoms interact as if they were spherical is a poor approximation for some elements. This paper outlines some of the progress being made in developing anisotropic atom-atom potentials, which can represent the effects of lone pair and pi-electron density on intermolecular interactions. It is difficult to determine the form of an atom's anisotropy empirically, and so it has to be derived from the molecular charge distribution, using recently developed theories of intermolecular forces. This can be done for each major contribution to the intermolecular potential for small polyatomics, resulting in more accurate intermolecular potentials. For organic molecules, at the moment, only the electrostatic contribution can be routinely described in this way, through a distributed-multipole analysis. However, computational studies using such an accurate electrostatic model, in conjunction with simple approximations for the other contributions, have been useful for understanding the structures of van der Waals complexes, biochemical interactions and molecular crystal structures. The development of new computer codes is gradually allowing anisotropic atom-atom potentials to be used routinely for an increasing range of types of simulation. Nevertheless, it will often be desirable, and adequate, to approximate an accurate potential by a simpler isotropic site-site form, with additional sites representing the anisotropic features. Assuming the isotropic atom-atom potential, without careful consideration of the distribution of charge in the molecule, can lead to problems in deriving quantitatively adequate potentials for many molecules and can even lead to conceptual problems.