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We describe an accurate periodic boundary condition (PBC) called the symmetric PBC in the calculation of the magnetostatic interaction field in the finite-differentiation-method fast-Fourier-transform (FDM-FFT) micromagnetics.The micromagnetic cells in the regular mesh used by the FDM-FFT method are finite-sized elements,but not geometrical points.Therefore,the key PBC operations for FDM-FFT methods are splitting and relocating the micromagnetic cell surfaces to stay symmetrically inside the box of half-total sizes with respect to the origin.The properties of the demagnetizing matrix of the split micromagnetic cells are discussed,and the sum rules of demagnetizing matrix are fulfilled by the symmetric PBC.