RCD+ intro
Random Coordinate Descent (RCD) is a versatile loop closure tool to efficiently generate loop ensembles. The algorithm solves the loop closure problem by optimizing randomly selected bonds and updating loop conformations using spinor matrices (geometric algebra). At download section you can get the latest version of RCD. Additionally, an online protein loop prediction server is available at rcd.chaconlab.org.
New 2019 version available! (v1.40)
References
Original method (loop sampling)
Chys P and Chacón P (2013). Random coordinate descent with spinor-matrices and geometric filters for efficient loop closure. J. Chem. Theory Comput. 9:1821-1829. 
Server method (include refinement)
López-Blanco JR, Canosa-Valls AJ, Li Y, and Chacón P (2016). RCD+: Fast loop modeling server. Nucl. Acids Res. 44 (W1): W395-W400.