Aliphatic diisocyanates are key ingredients in polyurethanes and are widely used in polyurethane processing. Trimers obtained by reacting three diisocyanates are very stable and exhibit excellent mechanical [1] and optical [2] properties. Despite this, they are mainly used as crosslinkers in coatings and adhesives since a full exploitation of their features is limited by processing issues. While the viscosity of the monomers is low and comparable with water, the trimers have a viscosity several orders of magnitude higher, making difficult their handling. From a fundamental point of view there is no clear understanding of this behavior. In this work we use quantum mechanical and atomistic simulations to understand and predict the properties of diisocyanates molecules. Starting from the GAFF force field, an improved set of parameters was introduced and tested in molecular dynamics simulations to calculate their static and dynamic properties. We show how the customization of force field parameters is fundamental to obtain reliable predictions of the properties of organic liquids. Viscosities of diisocyanate trimers ranging from 3 to 7 aliphatic carbons per monomer were calculated using Green Kubo method and indirect estimations for lower temperatures, and compared with experimental values. The results are in very good agreement with experiments [3], with simulations predicting the correct trend of viscosity with temperature and size of the studied molecules. We attribute the high viscosity of this molecules to the interaction of NCO terminations with the trimer NCO ring, as indicated by quantum mechanical calculations. [1] Moritsugu, M.; Sudo, A. and Endo, T., J. Polym. Sci. Part A: Polym Chem. 2013, 51, 2631–2637 [2] Flipsen, T. A. C; Steendam, R; Pennings, P. J., and Hadziioammou, G, Adv. Mat. 1996, 8(1), 45-48 [3] Driest, P. J., Lenzi, V., et al., Polym. Adv. Technol. 2016, doi: 10.1002/pat.3891