[gmx-users] Unexpected results arising from T- and P-coupling methods

[chris.neale at utoronto.ca]

I can't tell you if there is a problem or not.

The only intended difference that I can see is that for 4.5.X:

# grompp by default sets the new nsttcouple parameter equal to  
nstlist, this means T-coupling is done less frequently; grompp checks  
if tau_t is large enough
# grompp by default sets the new nstpcouple parameter equal to  
nstlist, this means P-coupling is done less frequently; grompp checks  
if tau_p is large enough

(see http://www.gromacs.org/About_Gromacs/Release_Notes/Versions_4.5.x )

Note that there were some fixes to P-R scaling in the 4.0.X series --  
se http://www.gromacs.org/About_Gromacs/Release_Notes/Revisions_in_4.0

To test this further, I suggest that you need to define a good  
procedure to ensure that what you are seeing is (b) from a well  
equilibrated system and is also (b) statistically significant.

I suggest that one way to do this is to cycle through your T- and P-  
coupling options at a given temperature. For example, at T=500 K, run  
X ns of Berendsen, then X ns of Parrinello-Rahman, then back to  
Berendsen, and so on for a few cycles. Each time you switch coupling  
method, be sure to use the structure output from the previous run.  
Also be sure that X ns is as long as you can afford. This way, you  
will be able to pick out systematic changes as temperature/density  
oscillations with periods that are related to your changes of  
algorithm. This also ensures that what you are seeing is not simply an  
artifact of having a poorly equilibrated density in your initial  
structure.

You could also run the same cycle with Berendsen and V-rescale.

Chris.

-- original message --

Dear gmxers,
    According to my recent practice, we find that the Berensen methods  
for T- and P- coupling can yield reasonable averaged density as a  
function of temperature, but when the v-rescale method and the  
Parrinello-Rahman method are employed for T- and P- coupling, somewhat  
unexpected results (i.e. density at higher temperature is bigger than  
that at lower tempearature) are obtained. Generally, the latter setup  
is considered to be prefered to the former one in simulating realistic  
ensemble. I am using gmx-4.5.3, and previously I have also performed  
one similar work using 4.0 which can generate expected results using  
the latter setup. I wonder if this version 4.5.3 has some bugs in  
calculating T and P, and are they dealt with in 4.5.4? Please give me  
some hints.

  Yours sincerely,
  Chaofu Wu, Dr.
   ------------------
   Department of Chemistry and Materials Science, Hunan University of  
Humanities, Science and Technology, Loudi 417000, the People?s  
Republic of China (P.R. China)
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