# Questions regarding fix addforce and fix create/break bonds

Dear Lammps user’s

I am sorry for the simple questions but I am a relatively new user to lammps.
I have several questions regarding fix addforce and fix create/break bond .

1. I want to add a time dependent force which looks like F=Asin(omegat).Can I do it with:
variable ampX equal 5.0

variable forceX equal {ampX}*sin(dt*time*{omega})

fix 1 all addforce v_forceX 0 0

2. When I use fix create bonds and break bonds, the inner functions that create or break or in a post_intigrate() function so that means that the checks for creating/breaking bonds are done after there was a time step propagation and the contributions of all the forces are taken into account ? The reason I am asking this is that I want to use create/break bonds with the external force I mentioned before.

3. I want to change fix add create/break bonds so that the test if to create or break wont be a simple test against a uniform random number but against some quantity that depends on the energy of the system. When I tried implementing this change I got that two bonded atoms can have more then one bond between them. when looking at the normal create bonds I don’t see this behavior , perhaps I need to add another constraint such that each bonded atoms can only have one bond ? in the normal fix create bonds, can there be a situation that two particles share more then one bond between them ?

Thank you very much!

Taj

Dear Lammps user's
I am sorry for the simple questions but I am a relatively new user to
lammps.
I have several questions regarding fix addforce and fix create/break bond .

I want to add a time dependent force which looks like F=A*sin(omega*t).Can I
do it with:
variable ampX equal 5.0
variable forceX equal \{ampX\}\*sin\(dt\*time\*{omega})
fix 1 all addforce v_forceX 0 0

why don't you create a simple test case input (two atoms are
sufficient) and dump the forces?

When I use fix create bonds and break bonds, the inner functions that create
or break or in a post_intigrate() function so that means that the checks for
creating/breaking bonds are done after there was a time step propagation and
the contributions of all the forces are taken into account ? The reason I am
asking this is that I want to use create/break bonds with the external force
I mentioned before.

why don't you create a simple test case and see for yourself?

I want to change fix add create/break bonds so that the test if to create or
break wont be a simple test against a uniform random number but against some
quantity that depends on the energy of the system. When I tried implementing
this change I got that two bonded atoms can have more then one bond between
them. when looking at the normal create bonds I don't see this behavior ,
perhaps I need to add another constraint such that each bonded atoms can
only have one bond ? in the normal fix create bonds, can there be a
situation that two particles share more then one bond between them ?

why don't you just read the documentation more carefully?
http://lammps.sandia.gov/doc/fix_bond_create.html

axel.

For the addforce check, you can also simply output
the value of the variable with thermo output and check
that it’s value varies with time as you expect.

For Q2, the added force will certainly be included
with the time integration, so that its effect will be
included when bonds are created/broken.

For Q3, you seem to be saying that you modified
the code for bond create/break and now it does