[gmx-users] genion command

[diana p]

Dear gmx users,
I want to simulate 0.6M CaCL2 aqueous solution. To add the ions I use
genion command and add 720 CL and 360 Ca ions respectively to the system of
pure 12568 water molecules.But on running the command genion I get error
message:

Program genion, VERSION 4.6.5
Source code file: /build/buildd/gromacs-4.6.5/src/tools/gmx_genion.c, line:
89
Fatal error:
No more replaceable solvent!

How should I add ions in the system.I would be more than pleased if someone
could guid me.
Thank You in advance