The reaction network is expanded using an iterative, flux-based approach in a core-edge model, where critical species and reactions are initially stored in the core and all possible new species that can be formed are stored in the edge. In its simplest form, species are moved from the edge into the core when the rate of formation, R i , for species i exceeds a user-defined fraction, , of the characteristic flux of the core, R char , seen in Equation (2).