This is the X-ray part of FMLSQ program intended to be used for Full-Matrix Least-SQuares refinement of macromolecules. Hence the name - FMLSQ. Written exclusively in FORTRAN 90 language with extensive usage OpenMP for speedy calculations. Unfortunately, we still need add geometry and some crystallographic restraints to make this program available for world-wide crystallographic community. The theory of the method is described in: Strokopytov, B. V. (2009). J. Appl. Cryst. 42, 1020-1029.