bookmark ARP/wARP Builds a protein model into an electron density map. ARP/wARP facilitates model building by initially interpreting a density map with free atoms of unknown chemical identity; all structural information for such chemically unassigned atoms is discarded. It consists of a number of tasks that are performed in an iterative fashion. Free atoms are used to obtain better electron density maps through refinement. The tool is based on the paradigm of viewing model building and refinement as one unified procedure for optimizing phase estimates.