FIRE++ is a header-only C++ implementation of the Fast Inertial Relaxation Engine (FIRE) algorithm for constrained/unconstrained numerical minimization. The algorithm is described in detail in Structural Relaxation Made Simple by Bitzek et al., Phys. Rev. Lett. 97 (2006).
Since FIRE++ is a header-only library, it requires no compilation. Its only dependency, Eigen, is also implemented as a header-only library.