A Courant condition-free modified partially implicit method. Revised electromagnetic particle code PS2M for bounded plasmas with conducting walls. The distortion of the dispersion of the light waves or the nonphysical frequency alias is shown to exist in the Courant condition relaxed methods already proposed. A new method for correcting this distortion without generating the alias is proposed. The proposed method is obtained by merging the merit of one type of method (the spectral method) to another type of method (the difference method).