def METHOD_NAME(self, k=None):
    """
    Calculates eigenstates and eigenvalues of the TDHF problem.
    Args:
        k (tuple, int): momentum transfer: either an index specifying the momentum transfer or a list of such
        indexes;
    Returns:
        Positive eigenvalues and eigenvectors.
    """
    if k is None:
        k = numpy.arange(len(self._scf.kpts))
    if isinstance(k, int):
        k = [k]
    for kk in k:
        self.e[kk], self.xy[kk] = self.__kernel__(k=kk)
    return self.e, self.xy