def  dBinaryRochedz(r, D, q, F):
    """
    Computes a derivative of the potential with respect to z.

    @param r:      relative radius vector (3 components)
    @param D:      instantaneous separation
    @param q:      mass ratio
    @param F:      synchronicity parameter
    """
    return -r[2]*(r[0]*r[0]+r[1]*r[1]+r[2]*r[2])**-1.5 -q*r[2]*((r[0]-D)*(r[0]-D)+r[1]*r[1]+r[2]*r[2])**-1.5