ANISOTROPIC ATOM-ATOM POTENTIALS

The forces between polyatomic molecules are traditionally represented by the isotropic atom-atom potential model. However, the implicit assu mption that the atoms interact as if they were spherical is a poor app roximation 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 intermol ecular interactions. It is difficult to determine the form of an atom' s anisotropy empirically, and so it has to be derived from the molecul ar charge distribution, using recently developed theories of intermole cular forces. This can be done for each major contribution to the inte rmolecular 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 stu dies using such an accurate electrostatic model, in conjunction with s imple 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. N evertheless, it will often be desirable, and adequate, to approximate an accurate potential by a simpler isotropic site-site form, with addi tional sites representing the anisotropic features. Assuming the isotr opic atom-atom potential, without careful consideration of the distrib ution of charge in the molecule, can lead to problems in deriving quan titatively adequate potentials for many molecules and can even lead to conceptual problems.