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?! -- Gromacs Users mailing list * Please search the archive at http://www.gromacs.org/Support/Mailing_Lists/GMX-Users_List before posting! * Can't post? Read http://www.gromacs.org/Support/Mailing_Lists * For (un)subscribe requests visit https://maillist.sys.kth.se/mailman/listinfo/gromacs.org_gmx-users or send a mail to gmx-users-requ...@gromacs.org.