Adsorption ONLY inside CNTs by LAMMPS

Hi,
i am reading some papers that investigate the adsorption isotherms of water inside CNTs. they use PBC at the axis of the tube so they consider as a volume only the interior of the tube. My question is that they all use GCMC and not MD. Since i am not currently familiar with GCMC (and atom deletion/addition that they perform), how can i do such a thing with LAMMPS and MD simulations? i mean given a volume and a pressure i will put an initial number of molecules h2o or co2 or ch4 for example. how can i do the analogous with lammps so to compute the adsorbed molecules and not all the molecules that i would have put in first place? is there any technique or i should use GCMC or consider an external space around the tube and place my molecules around the tube and see what will happened in the interior?

Regards
Jim

2011/10/7 Jim Jim <[email protected]>:

Hi,
i am reading some papers that investigate the adsorption isotherms of water
inside CNTs. they use PBC at the axis of the tube so they consider as a
volume only the interior of the tube. My question is that they all use GCMC
and not MD. Since i am not currently familiar with GCMC (and atom
deletion/addition that they perform), how can i do such a thing with LAMMPS
and MD simulations? i mean given a volume and a pressure i will put an

no. at least, not yet and not easily.

initial number of molecules h2o or co2 or ch4 for example. how can i do the
analogous with lammps so to compute the adsorbed molecules and not all the
molecules that i would have put in first place? is there any technique or i
should use GCMC or consider an external space around the tube and place my
molecules around the tube and see what will happened in the interior?

please read up on what a grand canonical ensemble represents
and you can answer your questions easily yourself.

axel.

And you can look at the fix gcmc command recently
added to LAMMPS by Paul Crozier, though it only
currently works for atoms not molecules (e.g. real water).

Paul is CCd.

Steve

It would be wrong if i used a finite sized CNT and to add two reservoirs in the two endings connected with PBC (so in practice one) ?

Have you read up on GCMC rules? I wouldn't try to re-invent a
well-established method. The tricky part for LAMMPS is
the insert/delete. Paul can advise you on what is possible
now, and what you could add.

Steve

I'd recommend you use GCMC for this. Please see what LAMMPS can do in this regard with fix GCMC (see the documentation). Steve is correct that fix GCMC cannot (yet) insert molecules (only atoms for now). You might consider using a one-site water model to get started, although the results probably wouldn't be very comparable to experiments. Alternatively, you could write your own version of fix GCMC that inserts molecules (or wait for someone else to do it). Or you could find a non-LAMMPS GCMC option.

I wouldn't recommend the reservoir idea because it would likely be challenging to deal with the new interfaces that would be introduced, you'd still have to establish the chemical potential (and pressure) of the reservoirs, and the computational cost would go up considerably. I'd recommend sticking with the established GCMC protocols.

Paul