[lammps-users] about mixture

Hi,

Your help is necessary to me. I have tabulated potential for a, b and a-b particles. How I can to create mixture concerning 50% - a particles and 50% - b particles?

With many thanks,
Aleksandr

Hi,

Your help is necessary to me. I have tabulated potential for a, b and a-b particles. How I can to create mixture concerning 50% - a particles and 50% - b particles?

alex,

please help yourself to reading:

http://lammps.sandia.gov/doc/create_atoms.html

with that you should be able to create a regular
lattice of A and B atoms with a 50-50 ratio.

if this is too regular, you can mix it up with a
heating/annealing cycle.

cheers,
   axel.

In addition to what Alex K posted, you can use
the set command to change some fraction of atoms
to a new type, randomly.

Steve

Hi,
I was wondering whether anybody has any idea about how to apply an external
force at a particular point/plane in space. I want to apply force on a
particular part of a polymer chain which is passing through a plane/point. I
looked up in the LAMMPS documentation but I got options for applying external
force to a group of atoms, etc but none for the situation I am looking for.

Regards,
JP

I think you'd have to write your own fix to do this.

Steve

Hi,
I want to change the group or atom or molecule ID of an atom or groups of atoms
in a space (if they are lying within a small box of dimension xlo, xhi, ylo,
yhi, zlo, zhi) dynamically. Since I want to do some operation on those groups of
atoms passing through this box. Any suggestions ?

Thanks in advance,
JP

Some part of my polymer molecule (please note only a part of the chain)
experiences a pulling force when passing through a particular region in space. I
want to apply a constant force on the polymer molecule passing through this
region. The only way to do is to change the group ID (dynamically) of that part
of the molecule which is passing through that region. How do I do that? Any set
or fix command?

    > Hi,
    > I want to change the group or atom or molecule ID of an atom or groups of
atoms
    > in a space (if they are lying within a small box of dimension xlo, xhi,
ylo,
    > yhi, zlo, zhi) dynamically. Since I want to do some operation on those
groups of
    > atoms passing through this box. Any suggestions ?

    so what is the question here?
    where is your problem?

    cheers,
      axel.

Some part of my polymer molecule (please note only a part of the chain)
experiences a pulling force when passing through a particular region in space. I
want to apply a constant force on the polymer molecule passing through this
region. The only way to do is to change the group ID (dynamically) of that part
of the molecule which is passing through that region. How do I do that? Any set
or fix command?

as far as i can tell, the best strategy would be to write a modified
version of fix addforce that accepts a region as argument.

doing this from the top lammps script level would be highly inefficient,
as you'd have to loop and undefine/redefine groups all the time and
could only do very short "run" periods.

cheers,
   axel.

Already tried that, it doesn't works. Therefore I tried modifying the existing
fix_add_force and it still doesn't works.

    > Some part of my polymer molecule (please note only a part of the chain)
    > experiences a pulling force when passing through a particular region in
space. I
    > want to apply a constant force on the polymer molecule passing through
this
    > region. The only way to do is to change the group ID (dynamically) of that
part
    > of the molecule which is passing through that region. How do I do that?
Any set
    > or fix command?

    as far as i can tell, the best strategy would be to write a modified
    version of fix addforce that accepts a region as argument.

    doing this from the top lammps script level would be highly inefficient,
    as you'd have to loop and undefine/redefine groups all the time and
    could only do very short "run" periods.

    cheers,
      axel.

    >
    >
    > > Hi,
    > > I want to change the group or atom or molecule ID of an atom or
groups of
    > atoms
    > > in a space (if they are lying within a small box of dimension xlo,
xhi,
    > ylo,
    > > yhi, zlo, zhi) dynamically. Since I want to do some operation on
those

"it doesn't work" is a completely insufficient description.

what doesn't work, how and why? i don't see any reason
for why a modified version of fix addforce should not work.

as far as i read the API, you'd need to store a reference
to the group class, and then instead of looping over the
local atoms and testing against the groupbit, you'd in
addition call the match() method of the respective region
class and compute/add/apply forces based on the return
value of that. there may be a few odds and ends to be fixed,
but that would be basically it.

in a more simple-minded implementation, you can just
set the region hardcoded in the fix.

cheers,
    axel.

You'd need to write your own fix to do this.

Steve

Axel sent me some code for this enhancement to
fix aveforce and fix addforce which I just posted as
a Aug 10 patch. Basically it applied the extra
force only if the atom is in a geometric region.

thanks Axel!

Steve

Hi,
  I want to know how to define a different value of dielectric constant for different regions of the simulation box if its possible.

Regards,
JP

Hi,
  I want to know how to define a different value of dielectric
constant for different regions of the simulation box if its possible.

it is not possible without changing the source code.
there are some tricky issues to consider, since
charged interactions are rather long ranged.

can you please provide some details of the kind of
system that you indent to model and what you expect
to get out of this?

thanks,
    axel.