The rotationally-averaged collision cross-section (CCS) represents the effective area for the interaction between an individual ion and the neutral gas through which it is travelling. CCS is an important distinguishing characteristic of an ion in the gas phase, being related to its chemical structure and three-dimensional conformation. CCS affects the mobility of an ion as it moves through a neutral gas under the influence of an electric field and ions may be separated accordingly using ion mobility spectrometry (IMS). CCS values may be measured experimentally using IMS. CCS values may also be estimated computationally if the 3D structure of the molecule is known.