[gmx-users] Conserved energy ("Conserved En.") in NVT simulation

[Janne Hirvi]

Dear gmx-users,

I have run some tests (especially) for pure water, being able to achieve pretty much perfect energy conservation in
NVE ensemble (with PME-switch and shifted VDW potential).

Then, just for the test, I continued to NVT ensemble by using previous .tpr and .cpt files, but the result was a quite
surprise, because there seems to be constant drift in "Conserved En.".

The drift (100kJ/mol = 0.3%) is not especially large, but it exists with v-rescale temperature coupling. On the other
hand, there is no drift at all, if I change to nose-hoover temperature coupling.

The behavior is similar also my other systems (liquid mixtures), even though then neither of temperature coupling
methods will give exactly perfect conservation, but nose-hoover gives much better one (500 kJ/mol = 3% vs. 60 
kJ/mol = 0.4%).

Actually, I was expecting to see no drift at all, after all systems behaved fine in NVE ensemble with same parameters
with an exception of added temperature coupling (especially when I used double precision for test purpose). 

So, the main question is, if it is just normal that even such parameters, which give perfect energy conservation in NVE
ensemble may introduce drift in "Conserved En.", when NVT ensemble is used? Or is it just purely (only reason for drift)
indication that some temperature coupling methods"work better" than others in this sense?

Regards,

Janne Hirvi