Hello - I'm trying to do mechanical properties on repeating carbon nanotubes I drew using Forcite, but the option is greyed out. I can select mechanical properties calculations when I do "perodic nanotubes", but let me explain what I'd like to do.

I want to draw a CNT with repeating units, then delete specific carbon atoms to simulate defects in the structure. Then compare the mechanical properties such as Young's modulus between a "pure" and "defect" carbon nanotube. 

Is there a way to "extend" the perodic nanotube structure so I can delete certain atoms and define a length? Thanks for any help!