def METHOD_NAME(self, frequency: float, coords: Coords) -> Tuple[complex, complex, complex]:
    """Main diagonal of the complex-valued permittivity tensor as a function of frequency.
    Parameters
    ----------
    frequency : float
        Frequency to evaluate permittivity at (Hz).
    Returns
    -------
    complex
        The diagonal elements of the relative permittivity tensor evaluated at ``frequency``.
    """
    if isinstance(self.medium, AbstractCustomMedium):
        return self.medium.eps_diagonal_on_grid(frequency=frequency, coords=coords)
    return self.medium.METHOD_NAME(frequency=frequency)