A large number of DFT approaches, including classical GGA approximations, hybrid HF/DFT approaches, and more recent tau -dependent functionals, have been tested for olefin polymerization reactions at a cationic aluminum system, and their results have been compared with refined post-HF methods. The results show that hybrid density functional methods always outperform classical GGA approaches, and more recent tau -dependent functionals are not yet competitive, providing nonsystematic errors in the energy evaluations. The effect of the lengthening of the polymer chain from the "standard" ethyl group to a butyl group was found to be significant and hence has to be taken into account when predicting the molecular weight of polymers. A computational strategy to study olefin polymerization at Al systems is proposed.