cross sections determined in an ion mobility experiment are, without doubt,
dependent on ion shape. This has clearly been demonstrated, as in the
case of carbon clusters.
However, because experimental cross sections are the result of an ion-buffer
gas scattering process, it is not straightforward to understand how the
Cartesian coordinates of a theoretical candidate structure can be converted
into a collision cross section. How is the collision process best be described?
Is scattering elastic or inelastic? Early work on ion mobility theory
lead the way, indicating it is the momentum transfer collision integral
that is the quantity that should be calculated.
Unfortunately, these integrals are difficult to evaluate when dealing
with nonspherical systems and can only be calculated numerically. However,
we have found that simple projection cross sections are - after careful
calibration - in many cases, an excellent approximation to the collision
integral. The guidelines in the table below represent our present understanding
of which method is best used for a given system. PA denotes "projection
approximation" (i.e. the shadow of a structure) and is calculated
by a program developed in our group (sigma).[3,4,5]
TJ denotes a "trajectory" calculation on a 12-6-4 potential
(Lennard-Jones + ion-induced dipole) centered at the position of each
atom  and EHSS ("exact
hard sphere scattering") a trajectory calculation using hard spheres
centered at the position of each atom.
For TJ and EHSS the program mobcal can be obtained from Martin
Jarrold's group at Indiana University.
See also: "Theory/Analysis: Ion Mobility Theory"