The broad field of molecular collisions is one of considerable current
interest, one in which there is a great deal of research activity, both
experi- mental and theoretical. This is probably because elastic,
inelastic, and reactive intermolecular collisions are of central
importance in many of the fundamental processes of chemistry and
physics. One small area of this field, namely atom-molecule collisions,
is now beginning to be "understood" from first principles. Although the
more general subject of the collisions of polyatomic molecules is of
great im- portance and intrinsic interest, it is still too complex from
the viewpoint of theoretical understanding. However, for atoms and
simple molecules the essential theory is well developed, and
computational methods are sufficiently advanced that calculations can
now be favorably compared with experimental results. This "coming
together" of the subject (and, incidentally, of physicists and chemists
!), though still in an early stage, signals that the time is ripe for an
appraisal and review of the theoretical basis of atom-molecule
collisions. It is especially important for the experimentalist in the
field to have a working knowledge of the theory and computational
methods required to describe the experimentally observable behavior of
the system. By now many of the alternative theoretical approaches and
computational procedures have been tested and intercompared.
More-or-Iess optimal methods for dealing with each aspect are emerging.
In many cases working equations, even schematic algorithms, have been
developed, with assumptions and caveats delineated.