# [lammps-users] Modelling air molecules

Steve,

I am interested in studying my system in atmospheric air instead of vaccum at room temp and atmospheric pressure. I am thinking of modelling atmosphere by the volumetric compposition of gaes like 78% Nitrogen, 20%oxygen, 1%Co2 and some hydrogen around my system (say diamond which has only Carbon atoms). I believe Nitrogen and oxygen exist as molecules (N2) & O2 respectively at 300K. Which is the best potential for modelling C-N & C-O interactions?

Thanks
Ajit

Steve,

``````            I am interested in studying my system in atmospheric
``````

air instead of vaccum at room temp and atmospheric pressure. I am
thinking of modelling atmosphere by the volumetric compposition of gaes
like 78% Nitrogen, 20%oxygen, 1%Co2 and some hydrogen around my system
(say diamond which has only Carbon atoms). I believe Nitrogen and oxygen
exist as molecules (N2) & O2 respectively at 300K. Which is the best
potential for modelling C-N & C-O interactions?

before doing that,
please do some simple "back-of-the-envelope" calculations.

how large a diamond system are you planning to look at?

from that make an estimate of how large a vacuum volume
you would have in your simulation cell.

once you have that, assume you have an ideal gas and then
compute how many atoms/molecules would actually be in that
volume.

you will find that, for the most part using no air is about as good
as including the air and that you would need a _huge_ volume
to represent air in the correct fractions.

finally, unless you are expecting chemical reactions to happen
(and use a model that can handle it), the interaction model, specifically
for C-N and C-O interactions is probably pretty much irrelevant as
you will have mostly high kinetic energy collisions, unless you go
to high enough pressure and low enough temperatures to see
condensation.

the final advice is: check the literature.
the chances to find suitable potentials or a recipe to generate
them are much higher than asking on a list like this one.

cheers,
axel.