The challenge of capturing the self-assembly in the act. Unrestricted and unbiased MD simulations of the self-assembly of Octacid4 with its known guests are challenging to perform because their complexation times were estimated to be a few minutes (or hours for the bulkiest guest naphthalene) according to NMR experiments . These complexation times are many orders of magnitude longer than current MD simulation times that are on the order of milliseconds.