The subroutine create_force_constants is the bunch of the
subroutines treating the forces and the site-point symmetries. These
data arrays are rearranged to solve linear equations of a global
matrix. Then using the subroutine dgelss of the LAPACK
routine, force constants are obtained. The dgelss takes most
CPU time and memory area in fropho procedure. Therefore the force
constants are written to the FORCE_CONSTANTS file. The calculated
force constants contain the effect of the periodic boundary condition.