There is a way to remove modes which would have certain modal effective mass fraction. See the MODESELECT case control command in the MSC Nastran Quick Reference Guide . It has option to specify which modes to include in the dynamic solution via Mode selection option based on modal effective mass fraction (MEFFMFRA) criteria.
Going back to this, this is not entirely solved. What I really want is a reduced op2, in other words, I do not want low contributing modes to even be written in the op2. I have an example of a terribly bad model for which 95% of the 4,000+ modes in the frequency range of interest are meaningless. The full op2 is close to 1TB, while only 5% of it is useful. It doesn't look like MODESELECT can alter DISPLACEMENT(PLOT) =ALL can it?
