[lammps-users] fix spring command

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1

Hello,

In fix spring command, the point to which spring is tethered is fixed
if "tether" is chosen.

Do you know how to let the point move, i.e., its coordinates can
change in a specified manner
(for example, moving along a vector in a given speed)?

Thanks a lot!

Yong

2

Yong,

No, the current version of fix spring can't be used in
the manner you wish. But you could modify it without
too much difficulty to get it to do what you want. You
might also look at fix lineforce:

http://www.cs.sandia.gov/~sjplimp/lammps/doc/fix_lineforce.html

which may be adequate for what you want.

Paul

3

Dear list

i am using fix rdf command for finding the "g" value at contact

when i used the command like shown below

{fix 6 active rdf 50 rdf.out 6000 1 1} , i am getting huge values

for the flow on inclined plane (granular systems)

example : angle of inclination :25 degrees
    g(0.999929) = 179.0176 (when i put kn=200000 and gamman =
    57.5)

actually for crystalline systems we can expect a value of aroung 70 to 80

can somebody clear my doubts if you have used this command?

Thanks

regards

anki

4

I've never used rdf with granular systems, but I don't
know any reason it shouldn't work. You might look
in the fix_rdf.cpp file and make sure the normalizations
look correct for the system you are running. You could
add some instrumentation to print out raw counts and
see if you are getting numbers you expect.

Steve

5

Paul, Thanks!

Actually, I can get what I want in an alternative way. That is to let
a group of atoms move
along a vector in a given constant speed without making the group of
atoms rigid.

Do you think LAMMPS can do this?

Thanks!

Yong

6

Yong,

LAMMPS has several fixes that impose forces to groups
of atoms, but I don't see any that will set the
velocity of a group to a constant vector. Perhaps you
could use fix drag:

http://www.cs.sandia.gov/~sjplimp/lammps/doc/fix_drag.html

to apply a set force to your group of atoms.

Paul

Paul, Thanks!

Actually, I can get what I want in an alternative
way. That is to let
a group of atoms move
along a vector in a given constant speed without
making the group of
atoms rigid.

Do you think LAMMPS can do this?

Thanks!

Yong

> Yong,
>
> No, the current version of fix spring can't be
used in
> the manner you wish. But you could modify it
without
> too much difficulty to get it to do what you want.
You
> might also look at fix lineforce:
>
>

http://www.cs.sandia.gov/~sjplimp/lammps/doc/fix_lineforce.html

7

You can set the initial velocity of a group of atoms
to some value, then turn off the forces, and they
will continue to move with v. However that
is like they are moving as a rigid body.

Steve