Problem in running lammps when system is not charge neutral

Dear all,
I want to simulate the process of heterogeneous nucleation (containing ions (40 na+)) of supersaturated water vapor, and PPPM is used to compute the long-range coulombic interactions. 
Thus, net charge of my simulation system is not zero, there is a warning in lammps: system is not charge neutral, net charge=40. 
I do not know whether the result is available if I ignore the warning? What should I do? Does lammps can accurately simulate the system which is not charge neutral?
I will appreciate if you can give me some suggestions!
Best regards
Zhang Chao

You should not ignore the warning.
Physically it does not make sense to simulate a periodic system with non-zero charge density.
You need to add negative ions to your system (Cl-) until the net charge is 0.
This should be easy to do (see below)

If you are using one of these water models: TIP3P, TIP4PEW, or SPC/E, then you can find the Lennard-Jones parameters for both Na+ and Cl- in
Joung & Cheatham JPCB (2008)

For SPC/E water, use:


If you are familiar with moltemplate, there is an example of

The full example (with README files and pictures) can be downloaded with the examples at (It is in the folder examples/waterSPCE+Na+Cl/
(Several other simple water models are in the “common” subdirectory, and it’s not hard to add your own.)