Hi Leon, Rafal's got it right, I'm afraid. The easiest way to solve this problem is probably to modify your model so as to "push away" the modes with large momentum so that they have energies outside of the relevant window that you are concerned with. Unfortunately you can't just "throw away" the modes at large momenta, because this would make the scattering problem under-determined. Although one could maybe find the "best" solution to this under-determined problem using least squares or something, this is quite awkward and I'm not even sure it's valid. In order to properly "throw away" the unwanted modes, you would instead have to make them evanescent. Doing this is essentially the same as modifying your model so as to "push the modes away". Happy Kwanting, Joe