Hi guys, 

I need to make a simulation with SeSn crystal. I have just selected for this crystal Universal forcefield. Universal has mass number for Se and Sn, but there are not their masses. So, my simulation failed because the masses of these atoms are zero and the system does not calculate properties for zero-mass atoms.
At this point, How can I modify the properties, such as mass, of these atoms in the force field? 
Thanks