def Lorentz1d_der_x(x, *p):
    """
    function to calculate the derivative of a Gaussian with respect to x

    for parameter description see Lorentz1d
    """
    return 4 * (p[0] - x) * p[2] / p[1] / (1 + (2 * (x - p[0]) / p[1]) ** 2
        ) ** 2
