I think decreasing the time step is easy to do in Gromacs, but how about turning off the bond constraint? Do I need to modify the topology file? Or should I set the constraints=none in mdp file?
The use of bond constraints is linked to the force field one use. If the force field required for time-step 0.002 ps constraints on the hydrogen atoms. One can not remove the constraints on bond involving hydrogen, since that can make the simulation inaccurate and sometimes even unstable.