FUSE is a CSP code designed to assemble crystal structures of ionic solids from layers (defined as modules), which are themselves constructed from small blocks (sub-modules, containing up to four atoms) generated according to rules based on chemical knowledge. Modules are then assembled into full crystal structures by stacking them according to the rules based on either cubic or hexagonal packing (depending on the specific unit cell). FUSE has a range of nine possible actions (Fig. ) that it can use to modify a crystal structure during the BH search, varying in impact from local actions (swapping the position of two individual atoms or sub-modules) to a global action (randomly generating a new structure); the actions are chosen independently according to a fixed user-defined probability distribution; a full description is presented in the Methods section. For action 6, the unit cell is limited to be of the same size or smaller, for 7 it can be of any size, so long as it conforms to the limit of the number of atoms used within a run of FUSE.