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Re: [lammps-users] damp fix press/berendsen and fix langevin
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Re: [lammps-users] damp fix press/berendsen and fix langevin

From: Axel Kohlmeyer <akohlmey@...24...>
Date: Fri, 7 Jul 2017 10:56:38 -0400

On Fri, Jul 7, 2017 at 7:06 AM, ruiyan <r-yan14@...61...> wrote:
> Dear all,
> I have several questions related to fix press/berendsen command and fix
> langevin command.
> 1. In fix press/berendsen command, we need to specify Pdamp and modulus
> value. From lammps manual of 2016Nov17, the recommneded value for Pdamp is
> 1000 timesteps, but I don't know what modulus mean in this command? Do we
> need to specify modulus value according to the real modulus of the material
> we simulate? Or we just need to consider the product of Pdamp and modulus to
> make pressure relax in a reasonable rate?  If the latter one is  correct, is
> there a recommend value for modulus?

please have a more careful look at the documentation for fix
press/berendsen. all your questions are answered.

> 2. In fix langevin command,
> (1) how to specify Tdamp? I don't find a recommended value for it.

that is impossible to provide, because it depends on how you are using
fix langevin. you need a different Tdamp when you use fix langevin an
explicit solvent system or implicit solvent (and different solvents to

> (2) Especially how to specify scale values? I know that the mass and size of
> different atom types should be accounted for in the choice of ratio values,
> but how? Suppose I define two atom types in my simulation box. Condtion1,
> for type1, mass=1, size=1, and type2, mass=1, size=2, what are sacle values
> for type1 and type2?

again, this strongly depends on the kind of simulation you are doing,
you may not need to set it at all.
at any rate, you need to read the documentation carefully, if you have
trouble understanding the details, you probably also need to read up
on langevin dynamics in general.

please note, that the LAMMPS manual is a "technical documentation",
i.e. it will tell you what the commands and keywords stand for, but
its purpose is not to explain the physics or tell you exactly how to
do your research (and neither is it the purpose of the mailing list).
think of it being like the owner's manual of a car: it tells you where
all the switches, buttons and levers are, but you still have to go to
driving school to learn how to drive.


> (3) Condition2, for type1, mass=1, size=1, and type2, mass=2, size=1, what
> are sacle values for type1 and type2?
> (4) Condition3, for type1, mass=1, size=2, and type2, mass=4, size=1, what
> are sacle values for type1 and type2?
> Thanks and look forward to your reply.
> Rui
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Dr. Axel Kohlmeyer  akohlmey@...24...
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.