# Will nvt+press/berendsen represents correct npt ensamble

Dear All

I am running some simulations with nvt+ berendsen barostat, my pressure is controlled but i doubt if it is a correct npt ensamble?

Please if anybody can give a light to this?

I am not sure about pressure control, but I know for a fact that the Berendsen thermostat does not reproduce a canonical ensemble, so intuitively I would guess that the Berendsen barostat also does not reproduce the isobaric-isothermal ensemble. I cannot quickly find a source for this right now, but I would think it is mentioned somwhere in the references at the bottom for the fix_nh-page (http://lammps.sandia.gov/doc/fix_nh.html), or maybe in a book on MD.

2015-02-28 8:06 GMT+01:00 Mohit Garg <[email protected]...>:

Dear All

I am running some simulations with nvt+ berendsen barostat, my pressure is
controlled but i doubt if it is a correct npt ensamble?
Please if anybody can give a light to this?

If you want an npt ensemble, why not just use fix npt? All Berendsen-type
methods are just quick/simple/dirty tricks enforce a certain state (keep
average T/p around a desired value), but they do not enforce the correct
ensemble (which is more than just the correct average T/p). fix npt uses
the Nosé-Hoover method, which should be able to generate the correct
ensemble (although there are cases where it will fail, too, but this is
more complicated matter).

Kristof

when i am using npt and taking the window average over pressure i am getting the pressure to be 0.7 atm ±20, which is against my desired pressure of 1 atm. So, how should i proceed with correct pressure using npt.

2015-02-28 17:24 GMT+01:00 Mohit Garg <[email protected]...>:

when i am using npt and taking the window average over pressure i am
getting the pressure to be 0.7 atm ±20, which is against my desired
pressure of 1 atm. So, how should i proceed with correct pressure using
npt.

0.7 +/- 20 atm can still be 1 atm, though. Maybe better equilibration will
get you smaller fluctuations, maybe not. I don't know your system well
enough (only you do).

Kristof