Starting density of a polymer sample before relaxation


I’m building a polyurea model (using Amorphous Cell in Materials Studio) before relaxing the model using
LAMMPS-17Nov16 (‘fix npt’). I understand I should compare the density after relaxation to a published value. But if the density can decrease upon relaxation, how should I choose the initial density when
building the model? I realize this may not be a LAMMPS question, but I’m trying to choose a starting point
for the simulation. I’m still new to LAMMPS.