The outgrowing clusters were chosen as all the 5 molecule clusters, that can be constructed from collision by a single of the monomer molecules included in the simulation. However, due to the size of IT and IP, these are effectively treated as 2 molecules, i.e. we allow smaller clusters like the IT-IT, IT-IP, IP-IP to also be outgrowing clusters. This is because both IT and IP can be converted to two acid molecules by the simple reaction with water.