Trying to mimic biophysical experiment concentration

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Dear GROMACS users,

I have a question on the gmx genion. I am trying to mimic the biophysical condition of my experiment, which is (20 mM Tris-HCl, pH 8.0, and 100 mM NaCl). When I use gmx grompp ions.mdp I get the value Note that said system has non-zero total charges = -6.000 and suggest to me to neutralize the system with counter ions with a physical salt concentration. What is the appropriate - conc should i use to mimic my physical experiment condition?

Thank you.