Temperature coupling

You shouldn't couple just a handful of atoms seperately to a heat bath. Or you should at least not be surprised to see large fluctuations when you do. If you only have a few atoms, the temperature, depending on the average speed, will fluctuate a lot. When you have more atoms, it averages out. Moreover, coupling the temperature to a fair amount of atoms will only involve small changes on the speeds of the atoms, whereas coupling to  a few atoms may give considerable alterations and thus cause artefacts in  your simulation.  The best thing to do is to merge the ions and solvent groups and couple  this to the heat bath.  Cheers,  Tsjerk   --  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -- :) -- :)  Tsjerk A. Wassenaar, M.Sc. -- :)  Molecular Dynamics Group -- :)  Dept. of Biophysical Chemistry -- :)  University of Groningen -- :)  Nijenborgh 4 -- :)  9747 AG Groningen -- :)  The Netherlands -- :) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~