We present a numerical algorithm based on the bullet physics library to generate densely packed (39% - 41%) structures of high-aspect-ratio nanorods for finite element micromagnetic simulations. The coercivities mu H-0(c) of the corresponding Cobalt nanorod structures vary between 0.50T and 0.67T, depending on the overall orientation of nanorods, which is in good agreement with experimental results. The simulations make it possible to estimate the coercivity loss due to incoherent reversal processes (27%) as well as the gain due to shape anisotropy (59%). Our calculations show permanent magnets consisting of packed Co nanorods with an energy density product (BH)(max) of 83kJ/m(3). We estimate that this value can be increased to 103kJ/m(3) by increasing the packing density from 40% to 45%. Another way to optimize (BH)(max) is the usage of novel materials. By varying the anisotropy constant K-1 and the saturation polarisation J(S) we found lower limits for these parameters to reach a certain energy density product. To increase (BH)(max) to 160 kJ/m(3), K-1 and J(S) have to be in the order of 450kJ/m(3) and 2.25T, respectively. The thermal stability of this approach was verified by elastic band calculations. Cobalt nanorods with a diameter of 10nm and a height of 50nm are thermally stable at room temperature, but problematic at 900K. Doubling the nanorods' height to 100nm increases that limit considerably.