Particle displacement vector for this jump

        Args:
            cell_lengths (np.array(x,y,z)): Cell lengths for the orthogonal simulation cell.

        Returns
            (np.array(x,y,z)): dr

def dr( self, cell_lengths ):
        """
        Particle displacement vector for this jump

        Args:
            cell_lengths (np.array(x,y,z)): Cell lengths for the orthogonal simulation cell.

        Returns
            (np.array(x,y,z)): dr
        """
        half_cell_lengths = cell_lengths / 2.0
        this_dr = self.final_site.r - self.initial_site.r
        for i in range( 3 ):
            if this_dr[ i ] > half_cell_lengths[ i ]:
                this_dr[ i ] -= cell_lengths[ i ]
            if this_dr[ i ] < -half_cell_lengths[ i ]:
                this_dr[ i ] += cell_lengths[ i ]
        return this_dr