graphene structure collapse

hi,


i am working on graphene. i jus rolled one graphene sheet (looks like multiwall CNT) using lammps …but when atoms comes closer they get attracted and the structure collapse. temperature shoot up suddenly when i use NVT thermostat…but structure looks fine when use temp/re scale +nve . i use modified tersoff potential. when i zoomed the structure after simulation i can see 5 to 7 bonds from each carbon atom .


1. I thought when atoms comes too closer repulsion force must be greater than attraction … and thus it would maintain tube like structure…
2. is there some problem with my potential?
3. why structure looks fine when temp/descale command is used compared to NVT (where sudden shoot up in temperature occur and whole structure looks weird ) ?..


Any advice appreciated


regards
Ragesh
|

hi,


i am working on graphene. i jus rolled one graphene sheet (looks like multiwall CNT) using lammps …but when atoms comes closer they get attracted and the structure collapse. temperature shoot up suddenly when i use NVT thermostat…but structure looks fine when use temp/re scale +nve . i use modified tersoff potential. when i zoomed the structure after simulation i can see 5 to 7 bonds from each carbon atom .


1. I thought when atoms comes too closer repulsion force must be greater than attraction … and thus it would maintain tube like structure…

unless they are radicals with unpaired electrons
and then re-combine and release a lot of energy
in the process.


2. is there some problem with my potential?

hard to say. the only way to validate this would
be to set up a similar calculation that is so small
that you can run it with an ab initio or at least
semi-empirical method and compare.


3. why structure looks fine when temp/descale command is used compared to NVT (where sudden shoot up in temperature occur and whole structure looks weird ) ?..

because temp/rescale is completely unphysical
and can remove near infinite amounts of kinetic
energy, while nvt simulates a real coupling to
a heat bath. however, if you do a simulation in
vacuo neither is a correct model. only nve would be.

axel.