[gmx-users] strange temperature coupling problem

[Nickle Fan]

Dear Gromacs users:

I am having a rather strange problem in one of my simulation. The system
consists of 2000 water molecules embedded between two walls. The wall atoms
are position restrainted (force constant: 5000).

The system is first equiliburated under NVT (Berendsen thermostat, tau_t =
0.1ps, Tsystem = 300K) for a while. Then I coupled the wall atoms to 300K
(tau_t = 0.1 ps), and water to 300K (tau_t = 3000 ps). I found that during a
100ps simulation, the temperature of water rise continuously to 400K! (the
wall atoms are OK though). I tried to disable water temperature coupling by
set tau_t = 0 for water, but it does not help.

I tried NVE simulation from the same initial configuration, and it appears
that the temperature of water during the first 50 ps is about 300K. So we
can rule out the possibility of bad local contact in the system. Then what
could possibly goes wrong with the system?

I  am using PME (4th order), vdw-type = Shift, rvdw-switch  = 0, rvdw = 1.1,
rlist = 1.2. Time step is 1fs.

Any help would be greatly appreciated.

Nick Fan
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