def evaluate_fN(model, NHI):
    """ Evaluate an f(N,X) model at a set of NHI values

    Parameters
    ----------
    NHI : array
      log NHI values


    Returns
    -------
    log_fN : array
      f(NHI,X) values

    """
    # Evaluate without z dependence
    log_fNX = model.__call__(NHI)

    return log_fNX