@staticmethod
def fit_deriv(x, amplitude, x_0, fwhm):
    """
        Drude1D model function derivatives.
        """
    d_amplitude = (fwhm / x_0) ** 2 / ((x / x_0 - x_0 / x) ** 2 + (fwhm /
        x_0) ** 2)
    d_x_0 = -2 * amplitude * d_amplitude * (1 / x_0 + d_amplitude * (x_0 **
        2 / fwhm ** 2) * ((-x / x_0 - 1 / x) * (x / x_0 - x_0 / x) - 2 * 
        fwhm ** 2 / x_0 ** 3))
    d_fwhm = 2 * amplitude * d_amplitude / fwhm * (1 - d_amplitude)
    return [d_amplitude, d_x_0, d_fwhm]
