[gmx-users] Distance Restraints set for water-ion cluster in gas phase (vacuum)

[liuyong_1007 at dicp.ac.cn]

Dear gromacs usres,

I want to run nvt simulation of water-ion cluster (Na+(H2O)250) in gas phase (vacuum) at higher temperature ( bigger than 200 K). 
But the cluster will collapse at high temperature. So the distance restraints is used to restraint the water molecules and the ion Na+ not to keep together as a cluster.
But there are so many atoms to set the distance constraint in the topology. So how can I set set the distance constraint in the topology easilier?
How to set the distance restraint between the atoms and center of mass of the water-ion cluster? 

Thank you very much!
Yong Liu



liuyong_1007 at dicp.ac.cn