Dear Justin
Thanks for your answer.
Based on your answer, I will add following lines to ions.itp file in opls
force field:
[ moleculetype ];
molname nrexcl
ZN 1
[ atoms ];
id at type res nr residu name at name cg nr charge mass
1 Zn2+ 1 ZN ZN
1 2 65.37000
Is my manner true?