[lammps-users] Problem about the temperature with fix smd

Dear users,

Now I encounter one problem about the temperature when I use fix smd command.

I use “fix nvt” for group ‘A’ in my system and “fix rigid” for group ‘B’. If just do the equilibrium and full relaxation, there is no problem, the temperature is stabled at the desired temperature (110K) after a while.

However, once I turn on the “fix smd” for group ‘B’ and meanwhile the “fix rigid” is still working for group ‘B’. The temperature suddenly dropped down over 50%. I don’t know what’s the problem. Does anyone know what’s the reason? Your kind help would be most appreciated.

Yajie Lei

I'll let Paul comment on whether it makes sense to use fix rigid
and fix smd on the same atoms ...


Yajie Lei,

I don’t know of anyone who has tested “fix rigid” with “fix smd”, and it isn’t clear to me that it is a good idea or even a “safe” thing to do. So user beware.

It isn’t surprising that the temperature drops since neither fix implies that the temperature will remain constant. Use of “fix smd” doesn’t even guarantee conservation of energy. You’ll probably need to thermostat, and realize that the thermostat will be replacing energy lost via use of “fix smd”.

Please note the following paragraph from the documentation ( http://lammps.sandia.gov/doc/fix_rigid.html ):

“This fix uses constant-energy integration, so you may need to impose additional constraints to control the temperature of an ensemble of rigid bodies. You can use fix langevin for this purpose to treat the system as effectively immersed in an implicit solvent, i.e. a Brownian dynamics model. Or you can thermostat additional atoms of an explicit solvent directly.”