[lammps-users] Dear users

Dear users,
I want to know how can I imply the wall atoms to attached to its lattice site by a harmonic spring of constant k in lamps. I don’t want use wall/harmonic command because it gives an ideal smooth wall.
I read somewhere that I need to use the harmonic bounds. How can I do it. Do I need give a bound to each atoms. I would be grate full if you can share the input file command that is required.

Sincerely yours,
Abbas

A.Abouei Mehrizi

Please try out fix spring/self: https://docs.lammps.org/fix_spring_self.html

Dear Dr. Axel,

Thanks for your reply,
I have another question.

How I can choose the spring constant that the ratio of the root means square displacement of the wall atoms and the nearest neighbor distance be less than the Lindeman criterion for melting.

Sincerely yours,
Abbas

A.Abouei Mehrizi

Please try out fix spring/self: https://docs.lammps.org/fix_spring_self.html

That is impossible to say in this generality from remote without knowing more many more details about your simulation and your research. You probably will have to determine this empirically.

And please keep in mind that the strength of the force constant changes the properties of the wall as it is like adding a bond to each atom. If you want a less invasive tethering of the wall, you should look at fix spring instead.

Thanks for your response,
I like to simulate the shear flow force of water on a smooth graphene surface. So I create 4 graphene sheets which capable me to create some pattern on the surface in the future simulation. so I want to use the harmonic bound for graphene atoms rather than fix the force and gravity to zero, to get more realistic results. I read in different papers that the spring constant should be selected in a way that the the ratio of the root means square displacement of the wall atoms and the nearest neighbor distance be less than the Lindeman criterion for melting, e.g. the paper entitled “Shear flow near solids: Epitaxial order and flow boundary conditions”. but I don’t know how to imply it in lammps and on the graphene sheets.
can you help me though that?

A.Abouei Mehrizi

This is a question for the authors of that paper and/or your adviser/supervisor/tutor.
Please note that this is not a LAMMPS problem, but a science problem.

In my personal opinion, the most realistic way to keep multiple graphene sheets from drifting around is to define a group for each of them and then use fix spring to just (weakly) tether their center of mass (fix spring uses unwrapped positions to compute the center of mass) to a fixed location in space. …and that can be done for each of the x-, y-, and z- components selectively.
That will allow the sheets to have their “relative” interactions exactly as if without the tether.

And another remark: for atomic scale systems, gravity is irrelevant.