# Boundary Conditions for Twisting simulation.

Dear all,

I wish to twist a fiber consisting of hollow tubes of carbon atoms by rotating the atoms on the two ends. Is there any way to use periodic boundary conditions? As soon as twisting begins, the left and right hand ends wont match. If I dont use periodic BC’s wont the atoms on the ends bond to atoms from other tubes.

Daniel

Dear all,

I wish to twist a fiber consisting of hollow tubes of carbon atoms by
rotating the atoms on the two ends. Is there any way to use periodic
boundary conditions? As soon as twisting begins, the left and right hand

not with LAMMPS, you will have to write your own code for that.

ends wont match. If I dont use periodic BC's wont the atoms on the ends
bond to atoms from other tubes.

i don't understand this. if you have no periodic boundaries, atoms
from the opposing ends don't "see" each other. how should they be able
to bond?

axel.

Axel,

Thank you for this. I will explain what I mean by 'atoms bonding at the
ends': I don't mean atoms on one end bonding to atoms on the other. You
see, each end will consist of the ends of a number of CNTs. The end
atoms are not sp2 bonded like the bulk atoms so atoms in one tube could
bond with atoms in neighbour tubes (only at the ends). With periodic
BCs, we don't have this problem. We would rather not have this end
bonding. What is the best BCs to use to get around this is the
question?

Daniel

What about saturating the end carbon atoms with hydrogens then? Are you using reactive force field like AIREBO?

Thanks for this idea Vikas,

We are using a reactive force field type potential. Would our potential apply to C-H as well as C-C? We have pair_style and pair_coeff commands dealing with this. Would we need separate ones for the C-H bonds?

Could we not just hold the atoms on the ends fixed. So during equilibriation they are fixed and during twisting they are fixed from moving closer together???

Daniel

If you forcefield has C-H interactions included, I don’t see an issue ( I don’t know which particular force field you are using). There are many reactive force fields with different approaches to bond formation.

During equilibration around room temperature, I don’t expect any new bond formation (even without hydrogen atoms), if the edge carbon atoms between two CNTs are interacting at least using van der Waals type of interction. You can keep them moving or fix them. It should make a different, at least from the bond formation perspective. The overall CNT bundle structure might change depending upon if you use fix or no-fix boundary conditions on the edge atoms.

When you start twisting the edge atoms by certain angle during your twisting simulations (I am not sure you are going to do MM or MD), you can always use keep them constrained using fix setforce command.

I hope this helps.

Thanks again Vikas

We are using the modified REBO potential of: Pastewka L, Pou P, Perez R, Gumbsch P, Moseler M. Describing bond-breaking processes by reactive potentials: importance of an environment-dependent interaction range. Phys Rev B 2008;78(16):161402-1–4.

We have previously equilibrated at and carried out mechanical tests on cnts at a temp of 0.5 K. What do you think is a good temperature to use? I guess during twisting we would have a thermostat on?

The command for twisting – “fix move rotate” will increment a fixed rotation to each atom on the ends throughout the simulations so I suppose this should mean that atoms on the end can’t move relative to each other during the twisting?

Daniel

This is no problem either. Those atoms are only moved by fix move. The will only move as you tell them to.

Axel.