The viscosity of selected families of ionic liquids has been assessed at the molecular level as a function of the intermolecular interactions using Density Functional Theory together with Atoms-in-a-Molecule and Natural Bond Orbitals approaches. Large ion clusters were studied, with the energetics and topology of interactions being related with experimental viscosity data to infer nanoscopic mechanisms controlling the viscous behavior. The reported results exhibit important information on the molecular basis controlling viscosity, which would allow advancing in the development of low-viscous ionic liquids, through a judicious selection of ion pairs considering their interactions and molecular structure. (C) 2014 Elsevier B.V. All rights reserved.