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
