Sl. Price, ANISOTROPIC ATOM-ATOM POTENTIALS, Philosophical magazine. B. Physics of condensed matter. Statistical mechanics, electronic, optical and magnetic, 73(1), 1996, pp. 95-106
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.