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1

I use this command "fix 1 big rigid/nve/small molecule langevin 1.0 1.0 0.1 15346 " to create some rigid bodies,now my purpose is to observe the behavior of the rigid body in 2d simulation which means i need to keep erasing the Vz Fz of the atoms (or Fz Vz and Tz of the rigid body?It confuses me…).
I use command "fix setforce"and to make Fz=0,and use "velocity set NULL NULL 0 "to make the initial Vz=0.But the rigid body are not confined in 2d,they can movitation alone the z dimension.
And also i found that i can not use the command “fix enforce2d”,when i use that command the procedure broken.

what should i do to reach my purpose?

neigh_modify exclude molecule/intra big

ERROR: Illegal neigh_modify command (…/neighbor.cpp:2058)

And i dont know why this error arise.

Best
Tian.

2

I use this command "fix 1 big rigid/nve/small molecule langevin 1.0 1.0 0.1
15346 " to create some rigid bodies,now my purpose is to observe the
behavior of the rigid body in 2d simulation which means i need to keep
erasing the Vz Fz of the atoms (or Fz Vz and Tz of the rigid body?It
confuses me......).

please try without the langevin option.

I use command "fix setforce"and to make Fz=0,and use "velocity set NULL NULL
0 "to make the initial Vz=0.But the rigid body are not confined in 2d,they
can movitation alone the z dimension.
And also i found that i can not use the command "fix enforce2d",when i
use that command the procedure broken.

again, without the langevin option it should work.

what should i do to reach my purpose?

neigh_modify exclude molecule/intra big
ERROR: Illegal neigh_modify command (../neighbor.cpp:2058)
And i dont know why this error arise.

because you are using an old version of LAMMPS but look up the online
documentation. the online documentation *always* reflects the latest
development versions of LAMMPS.
as you can see here: http://lammps.sandia.gov/bug.html development
progresses and with that progress occasional backward incompatible
changes need to be made.

axel.

3

i have to thank you for your help in the very beginning. I am a new learner of lammps.Without your explain,it may take me more time to know lammps well.

And for the previous question,there still some thing that confuse me.

I use"fix langevin" in “atom_style ellipsoid” before,and in that simulation i use “fix setforce” “fix enforce2d”,the system confine in 2d,and operates smoothly.

why things go wrong for rigid body…

Another question,the command “fix setforce NULL NULL 0”,set Fz=0.Does the setting always working during the simulation?i mean,do the command set Fz to zero during the simlation?The particle only receive the force along x axis and y axis whatever other settings?If so,why i use command"langvein",and “fix setforce” doesnt work?Do the command “fix enforce2d” the same as the command"fix setforce"?

4

i have to thank you for your help in the very beginning. I am a new learner
of lammps.Without your explain,it may take me more time to know lammps well.

And for the previous question,there still some thing that confuse me.

I use"fix langevin" in "atom_style ellipsoid" before,and in that simulation
i use "fix setforce" "fix enforce2d",the system confine in 2d,and operates
smoothly.

why things go wrong for rigid body.....

because there is an issue with the fix rigid implementation that needs
to be corrected when using rigid bodies in 2d with the langevin flag.
i have submitted a pull request, with changes to address this
particular issue: https://github.com/lammps/lammps/pull/570

for 2d simulations, there should be no need to use fix setforce to
zero out the z-direction force component, fix enforce2d will do that
and more.

Another question,the command "fix setforce NULL NULL 0",set Fz=0.Does the
setting always working during the simulation?i mean,do the command set Fz
to zero during the simlation?The particle only receive the force along x
axis and y axis whatever other settings?If so,why i use
command"langvein",and "fix setforce" doesnt work?Do the command "fix
enforce2d" the same as the command"fix setforce"?

i don't have time to explain the entire logic of the LAMMPS source
code and its commands to you. in addition, you are making connections,
that do not exist or make sense. if you do want answers to these kind
of questions, you have to dig into the source code and figure it out
by yourself. what LAMMPS is supposed to do is documented in the
manual. i am happy to discuss if LAMMPS behavior differs from what is
documented in the manual and you provide easily reproducible proof.

BTW: please also note, that i *strongly* dislike the phrase "does not
work". it is vague, non-descriptive, and counterproductive. it can
mean anything from "LAMMPS crashes hard" (e.g. with a segmentation
fault), over "LAMMPS refuses to run my input deck" to "LAMMPS does not
produce the expected result" (which may actually be correct).
in the future, please be specific or else you will see your requests
for help ignored.

axel.

5

Thanks!I will submit question more specific next time!

best.
Tian.