Given electronic wave functions generated by the MCHF_CI program (LSJ format), this program calculates diagonal Landé g factors that determine separations of magnetic sublevels in weak external magnetic fields. In addition the program computes off-diagonal Landé g factors and constructs the total interaction matrix for an atom in a magnetic field. By diagonalizing the interaction matrix and plotting the eigenvalues as functions of the magnetic field, Zeeman structures beyond the weak field limit are obtained.