[gmx-developers] virial contributions

Berk Hess hessb at mpip-mainz.mpg.de
Mon Oct 16 14:07:33 CEST 2006

H.J.Risselada wrote:

> Dear GMX-devolopers
> When looking to the code pull.c (gmx-3.3.1) I see e.g that in the 
> umbrella sampling part a virial contribution is calculated from the 
> double sum of the pull forces together with the pulled positions 
> relative from equilibrium (the well of the potential). However, 
> instead of the expected factor -0.5 a factor of +0.5 is included while 
> in other parts of the code the -0.5 is included (calcvir.c).
> The pressure equations implemented in gromacs is the one directly from 
> litterature which however also implies the -0.5 factor in the virial.
> It's by sure not unthinkable that I just overlook a minus sign or a 
> substraction somewhere else on the code, but on first sight it looks 
> to me the virial contribution of pulling is now substracted instead of 
> added to the total_virial.
> Can someone please explain me this ?
> Physically I would expect the opposite, hence it's the virial 
> contribution added to form the virial which represents the state of 
> the biased ensemble.
I thinks it should be correct.
I have added these virial contributions some time ago,
but do not remember the details (I should have added some more comment).
I would guess that dr has the opposite sign of what you think: it is 
x_ref - x,
not x - x_ref.


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