[lammps-users] fix smd couple problem

Dear all:
I am trying to add a spring between two atoms and incrementally push the two atoms away from each other. Initially, they are covalently bonded and by adding an extra spring, I want to finally break the bond. I used command " fix smd" to do this. I just did a simple test. I have two atoms and put them in a distance of R0. Then I use
" fix pushing atom1 cvel 1000.0 0.0001 couple atom2 auto auto auto R0 " to push the atoms. I expected the two atoms will be pushed away gradually by a velocity of 0.00001. However, I found at the very first of the simulation, the two atoms are just been pushed away by a huge distance (like a explosion). I do not know what is the problem. Do anyone have any idea about this? Thanks a lot.

Regards
Xu

Xu Huang
Engineering Science and Mechanics
Penn State University, PA, 16801

2011/3/7 huangxu <[email protected]...>:

Dear all:
I am trying to add a spring between two atoms and incrementally push the two
atoms away from each other. Initially, they are covalently bonded and by
adding an extra spring, I want to finally break the bond. I used command "
fix smd" to do this. I just did a simple test. I have two atoms and put
them in a distance of R0. Then I use
" fix pushing atom1 cvel 1000.0 0.0001 couple atom2 auto auto auto R0 " to
push the atoms. I expected the two atoms will be pushed away gradually by a
velocity of 0.00001. However, I found at the very first of the simulation,
the two atoms are just been pushed away by a huge distance (like a
explosion). I do not know what is the problem. Do anyone have any idea about
this? Thanks a lot.

the r0 that you set is not the r0 that you think it is. just set it to
zero and try again.

axel.

Hi Axel:
Thank you for your reply. I just tried r0=0 and I had the same problem. I do not know why.
Remember, in my system, atom1 and atom2 are covalently bonded already and if they overlap with each other, they will be a huge force applied on each atom.
I do not know what is R0.

Regards
Xu

2011/3/8 huangxu <[email protected]...>:

Hi Axel:
Thank you for your reply. I just tried r0=0 and I had the same problem. I do
not know why.
Remember, in my system, atom1 and atom2 are covalently bonded already and if
they overlap with each other, they will be a huge force applied on each
atom.
I do not know what is R0.

please provide a simple, easy to run example input.

it is very difficult to discuss these matters without having
something to look at. i always feel like a blind person having
to discuss colors. rather than having to imagine what you
are seeing, it would prefer to see it for myself.

axel.