nlgeom and master degrees of freedom

[4WillyWonka]

Hi Everyone

I want to make a modal analysis of a structure with hyper material and I don't know how to activate "nlgeom" that asks me when I do the analysis. I only know activate it when I make static analysis.

I want to know too which are the master degrees of freedom of a structure in 3D whose low surface is completely constrainst and in the top surface an impulsive force is applied.

Thank you very much. I hope you understand me. Jorge.