not zero net charge

hi all
I want to simulate a tio2 nanotube that the net charge is not zero. how can I resolve this problem?
is qeq method a good choice? what approaches can I use?

what is the exact value of the net charge?

my net charge is +21

that means that your net charge is most likely due to how you are constructing the geometry of the tio2 nanotube. probably because of polar surfaces.
if you want a neutral system, you have to adjust the geometry accordingly.