The Modal Analysis add-on for RFEM 6 / RSTAB 9 supports you when determining natural frequencies and mode shapes.
Another useful program is the Response Spectrum Analysis add-on for RFEM 6 / RSTAB 9. Here, you can perform seismic analyses using the multi-modal response spectrum analysis. Among others, you can find the Swiss standard SIA 261 implemented in the add-on.
Modal Analysis Add-on for RFEM 6 / RSTAB 9 Response Spectrum Analysis Add-on for RFEM 6 / RSTAB 9