def dre_drho0(self, pars):
        r""" Compute partial derivative of real parts with respect to
        :math:`\rho_0`

        :math:`\frac{\partial \hat{\rho'}(\omega)}{\partial \rho_0} = 1 -
        \frac{m (\omega \tau)^c cos(\frac{c \pi}{2}) + (\omega \tau)^c}{1 + 2
        (\omega \tau)^c cos(\frac{c \pi}{2}) + (\omega \tau)^{2 c}}`

        Note that partial derivatives towards :math:`\rho_0` are 1D, in
        contrast to the other parameter derivatives, which usually return 2D
        arrays!

        Returns
        -------
        dre_drho0: :class:`numpy.ndarray`
            Size N (nr of frequencies) array with the derivatives

        """
        self._set_parameters(pars)
        numerator = self.m * self.otc * (np.cos(self.ang) + self.otc)
        term = numerator / self.denom
        specs = np.sum(term, axis=1)

        result = 1 - specs
        return result