# Stress calculation for nano-structures

Dear All,

In my Lammps simulation, I have a single wall nano-tube, with non-period boundary conditions for all 3 dimensions.

Then I used “compute pressure command” to get the stress tensor: http://lammps.sandia.gov/doc/compute_pressure.html

My question is what volume “V” is used in Lammps for this calculation? Since the nano-tube is different than the bulk material, so I am not clear how to get the proper volume that is needed in the stress equation.

Lili

Dear All,

In my Lammps simulation, I have a single wall nano-tube, with non-period
boundary conditions for all 3 dimensions.

Then I used "compute pressure command" to get the stress tensor:
http://lammps.sandia.gov/doc/compute_pressure.html

why not use compute stress/atom and then compute/reduce sum? it is
mentioned on the same documentation page even.

My question is what volume "V" is used in Lammps for this calculation?

the volume used is the total volume of the system.

Since the nano-tube is different than the bulk material, so I am not clear how to
get the proper volume that is needed in the stress equation.

you do not need the volume to compute the virial stress. only for
pressure, which is not really meaningful to compute for an object like
a SWCNT.

axel.

Dear Axel,

I do use “compute stress/atom and then compute/reduce sum.” As the doc pages says, the values of this procedure has the unit of “stress * volume.” This is why I need to find a proper “volume” for the SWNT to get the stress tensor.

Thanks for explaining that Lammps takes the box volume as the volume of SWNT in the “compute pressure command.”

Many thanks,
Lili

Dear Axel,

I do use "compute stress/atom and then compute/reduce sum." As the doc pages

sorry, but you _explicitly_ said that you are using compute pressure.
just read the quoted test below. you cannot expect people to give a
correct answer, if you don't provide a correct description of what you
are doing.

says, the values of this procedure has the unit of "stress * volume." This
is why I need to find a proper "volume" for the SWNT to get the stress
tensor.

that is more a question of whether virial stress is the correct
property to look at. this has little to do with LAMMPS. please have a
look at the atoms to continuum package from some alternate
descriptions. this is a frequent discussion topic and i recommend
looking through the mailing list archives, too. as far as i recall,
there is no simple answer to this leading to a "perfect" solution, but
rather people are using different approximations. it all depends on
what kind of data you want to produce for what purpose and what level
significant systematic error in your calculation from using a
classical model.

Thanks for explaining that Lammps takes the box volume as the volume of SWNT
in the "compute pressure command."

again, that is an incorrect description. please be more careful. this
can be easily misleading, if you communicate with people that have
little experience in these matters. compute pressure always computes
the pressure for the entire system and doesn't care what is in it. in
fact, a pressure computation for a subsystem has the problem that the
required reference volume is often ill defined.

axel.