Thanks for your answer.

I used mode-superposition harmonic analysis and I now got a beautiful displacement vs. frequency plot. But to plot acceleration vs. frequency I want to multiply the displacement with its corresponding frequency. The nodal information is stored in a variable using NSOL,2,12,U,Y. I could create an array parameter for the frequency but I have not found a way to combine those two. How do I do this?

EDIT: I think I found the solution by using FILLDATA to fill variables