Strange static results of my using "fix addforce" command

Dear lammps users,
In short, I am dragging one simple polymer chain from curving state to straight state via the “fix addforce” command. The upper end of the chain was fixed by “setforce 0 0 0”, the other lower end was pulled down via " fix addforce" command. However, my result displayed by VMD shows strangely that only the atoms of the lower end moved down quickly, while the other part which looks still kept the original shape which had little change for all the computational process.
I have several tries. (1) change the force field from CVFF to PCFF, (2) change the integration ensemble from NVE to NVT, (3) change the length of the chain. All of tries give me the same strange results as described above, i. e. the curving chain didn’t became straight under the “addforce”, which could be seen by opening the test20141104.lammpstrj in the enclosure via VMD. Besides, my data.file and input script are also enclosed. I am looking forward to your advice and comment. ( NOTE THAT: If you run the in.tensile, please put both of the rub-chain.data and the in.tensile in a new folder because one hundred dump.drag.*cfg files. I have to dump the cfg file before dumping the lammpstrj, otherwise the computational process would stop with “ERROR: Cound not find dump_modify ID (…/output.cpp:596)”. I don’t know why. If you know the reason, any hint is welcome.)
For quick snapshot of my input script, the loading part is as follows,

--------------#boundary#------------------

fix 1 all nve
fix 2 upper setforce 0.0 0.0 0.0

--------------#drag#------------------

fix 4 lower aveforce 0 -500 0

#fix 4 lower drag 0 -1500 0 5 2
#fix 4 lower move linear 0 -1 0

--------------#dump#------------------

dump 1 all cfg 200 dump.drag.*.cfg mass type xs ys zs vx vy vz fz

dump 10 all custom 200 1.lammpstrj id type xu yu zu
dump_modify 1 sort id

--------------#end of the part#---------

Best wishes!

D. Chai

test20141104.lammpstrj (46.8 KB)

rub-chain.data (36 KB)

in.tensile (2.26 KB)

Dear lammps users,
         In short, I am dragging one simple polymer chain from curving
state to straight state via the "fix addforce" command. The upper end of
the chain was fixed by "setforce 0 0 0", the other lower end was pulled
down via " fix addforce" command. However, my result displayed by VMD shows
strangely that only the atoms of the lower end moved down quickly, while
the other part which looks still kept the original shape which had little
change for all the computational process.

The force applied to group lower may simply be too large. Try smaller
forces or set a velocity to the group.

        I have several tries. (1) change the force field from CVFF to PCFF,

(2) change the integration ensemble from NVE to NVT, (3) change the length
of the chain. All of tries give me the same strange results as described
above, i. e. the curving chain didn't became straight under the
"addforce", which could be seen by opening the test20141104.lammpstrj in
the enclosure via VMD. Besides, my data.file and input script are also
enclosed. I am looking forward to your advice and comment. ( NOTE THAT:
If you run the in.tensile, please put both of the rub-chain.data and the
in.tensile in a new folder because one hundred dump.drag.*cfg files.

This is unnecessary since we can change the number of cfg outputs.

I have to dump the cfg file before dumping the lammpstrj, otherwise the
computational process would stop with "ERROR: Cound not find dump_modify
ID (../output.cpp:596)".

This does not make sense. You just have to make sure that dump_modify ID
is already declared by a dump.

Dear Ray,
Thank you for your advice. Yes, the force I add to the lower group is too large. In addition to the important reason, another critical reason is that my relaxation time is too short. After increasing the integral steps from 2000 to 200000, I am very glad to tell you that the polymer chain is straightened finally.
Best wishes!
D. Chai

在 2014-11-05 06:03:38,“Ray Shan” <[email protected]…24…> 写道: