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Re: [lammps-users] nonpriodic boundry condition
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# Re: [lammps-users] nonpriodic boundry condition

 From: Axel Kohlmeyer Date: Sun, 1 Oct 2017 15:18:11 -0400

On Sun, Oct 1, 2017 at 2:48 PM, Hossein Geraili wrote:

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On Sun, Oct 1, 2017 at 8:46 PM, Axel Kohlmeyer wrote:

On Sun, Oct 1, 2017 at 12:03 PM, Hossein Geraili wrote:
Dear lammps users,
How can I control the temperature in nonperiodic boundary condition without controlling volume with NVE?

​please note, that there is a significant difference between using fix nve and an NVE ensemble. you *can* achieve an NVE ensemble by using fix nve, but using fix nve does not automatically result in an NVE ensemble. this has been explained on this mailing list many, many times.

besides, fix nve does *NOT* control the volume. ​ in fact, to be compatible with creating an NVE ensemble is *MUST NOT* do *any* manipulations of the system, neither volume nor (kinetic) energy.
why in my simulation volum is constant?( nonperiodic and  fix NVE)

​if nothing changes the volume, it will remain constant.

however, you seem to be missing important conceptual points about what you are trying to do, and that is really very worrisome. you need tutoring in statistical thermodynamic beyond what a mailing list can provide.

for example, in a non-periodic system, the volume is effectively infinite (unless you create artificial boundaries through a wall fix). however, that is not practical to do with a simulation program using domain decomposition parallelization. so you can set ​it to any size you want, for as long as the atoms remain in that volume.
so you can use fixed boundaries with a safety margin ​or you can have the simulation program update the boundaries in such a way, as the ​box adjust to the extent of the included atoms (aka shrinkwrap boundaries). in any case, since you have no interactions with peridodic images the thermodynamic properties are the same (except pressure, which is not well defined under these circumstances and compute pressure computes the pressure for a periodic or bounded system).

axel.

​​fix nve is fully compatible with non-periodic boundaries.

I want to calculate density at the ideal gas state, so I need to control temperature, but thermostats have no time integration.

however, you are not making much sense here. using a thermostat for an ideal gas is pointless, since ideal gas particles do not interact; so how could they couple with a reservoir?

​also, there is not such thing as an "ideal gas state". an ideal gas is a well defined thermodynamic entity. there is little value to doing simulations of an ideal gas, since all its properties can be obtained analytically.

besides, what has this all to do with periodic-boundary conditions?

axel.

best regards

--
Hosein Geraili Daronkola
M.Sc.Graduated of physical chemistry
Physical Chemistry,
Department of Chemistry,
Sharif University of Technology.
geraili_hosein@...7110...

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--
Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.

Thanks

--
Hosein Geraili Daronkola
M.Sc.Graduated of physical chemistry
Physical Chemistry,
Department of Chemistry,
Sharif University of Technology.
geraili_hosein@...7110...

--
Dr. Axel Kohlmeyer  akohlmey@...92......  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.