[gmx-users] parrinello-rahman to wrong pressure

[Michel Cuendet]

Hi gmx fans,

   After heating up my system with a slightly oversized fixed box, I 
start the equilibration with a barostat (parameters below). How is it 
possible that parrinello-rahman expands the box and maintains an average 
pressure of about -1900 bar (over 100's of picoseconds) ? All conditions 
identical, the berendsen barostat decreases the box size and then keeps 
a reasonable pressure. It seems that the final pressure for 
parrinello-rahman depends on the size of the system : with more water, 
it reaches a pressure of -2400 bar.

   Has anyone an explaination ?

   Thanks,

Michel


tcoupl                   = nose-hoover
tc-grps                  = Protein
tau_t                    = 0.1
ref_t                    = 300.0
Pcoupl                   = berendsen  /   parrinello-rahman
tau_p                    = 1.0
compressibility          = 4.5e-5
ref_p                    = 1.0




   PS: My previous message about thermostats in presence of a frozen 
group has still not been replied.