All calculations were carried out with the frozen core approximation. For the damped calculations, a convergence threshold of 10 -4 was applied for both the linear equations and the numerical Laplace transformation; the qualitative results were found to be rather insensitive to the threshold. In the resonant case, the residuals of the eigenvalue and linear equations were converged below thresholds of 10 -8 and 10 -6 , respectively.