AIREBO potential cutoff

Dear Lammps Users,

I am using AIRBEO potential for Carbon nano tube-tube interaction. I am confused about cutoff radius for LJ interaction between C-C atoms. In LAMMPS documents I see this part " In the standard AIREBO potential, sigma_CC = 3.4 Angstroms, so with a scale factor of 3.0 (the argument in pair_style), the resulting E_LJ cutoff would be 10.2 Angstroms" .

What this scale factor “3” coming from that makes the cutoff radius equal to 10.2 Angestrom?

I refereed to " Stuart, Tutein, Harrison, J Chem Phys, 112, 6472-6486 (2000)" but I could not understand why 10.2 Angstroms for LJ cutoff.

I really appreciate if anybody give me a clue.

Thanks,

Samaneh

Dear Lammps Users,

I am using AIRBEO potential for Carbon nano tube-tube interaction. I am
LAMMPS documents I see this part " In the standard AIREBO potential,
sigma_CC = 3.4 Angstroms, so with a scale factor of 3.0 (the argument in
pair_style), the resulting E_LJ cutoff would be 10.2 Angstroms" .

What this scale factor "3" coming from that makes the cutoff radius equal to
10.2 Angestrom?

I refereed to " Stuart, Tutein, Harrison, J Chem Phys, 112, 6472-6486
(2000)" but I could not understand why 10.2 Angstroms for LJ cutoff.

I really appreciate if anybody give me a clue.

making a choice?
typically, their choice is a compromise between accuracy and
computational efficiency.

axel.

Dear Axel,

According to (Stuart), r_cc_max_LJ = 1.12 sigma. sigma = 3.4 Angestrom. if rij > r_cc_max_LJ pair distance has no effect on LJ intraction . other criteria are connectivity and bond strength. I am trying to understand that where is the scale factor "3" is coming from that makes cutoff = 10.2 Angestroms.

Bests,

Dear Axel,

According to (Stuart), r_cc_max_LJ = 1.12 sigma. sigma = 3.4 Angestrom. if rij > r_cc_max_LJ pair distance has no effect on LJ intraction . other criteria are connectivity and bond strength. I am trying to understand that where is the scale factor "3" is coming from that makes cutoff = 10.2 Angestroms.

please explain, why this matters? you could just as well ask why it
uses a LJ functional form and not Morse. or any other choice made.

as i already explained, this is a choice of the specific
parameterization. In other words this is the setting for which the
potential file bundled with LAMMPS has been parameterized. This is
also mentioned in the documentation for pair_style airebo. please also
note, the 10.2 angstrom you quote only applies to C-C interactions.
for other elements with different sigma, you get different cutoffs.

axel.

Understanding the cut-off matters because when I place two identical CNT(15,15 ) with 80 Angestrom length close together with 10 Angestrom (according to AIRBEO LJ cutoff is 10.2 Angesom) distance between them and minimize(CG) the system, I expect to see some attraction between them. But nothing happened. Indeed Attraction beween two CNTs happens when the distance between them is less than 7.8 Angestroms. I am using these commands for airebo potential.

pair_style airebo 3.0
pair_coeff * * CH.airebo C

minimize 0 1e-4 10000 10000

My question is that if cutoff is 10.2 Angestrom, why two CNTs attract each other if the distance between them is below 7.8 Angestrom.

Thanks,

Understanding the cut-off matters because when I place two identical CNT(15,15 ) with 80 Angestrom length close together with 10 Angestrom (according to AIRBEO LJ cutoff is 10.2 Angesom) distance between them and minimize(CG) the system, I expect to see some attraction between them. But nothing happened. Indeed Attraction beween two CNTs happens when the distance between them is less than 7.8 Angestroms. I am using these commands for airebo potential.

now, why didn't you tell this before?
with this description you have turned a cryptic and mostly meaningless
inquiry into a valid question.
lesson: answers are only as good as the question. thus better

pair_style airebo 3.0
pair_coeff * * CH.airebo C

minimize 0 1e-4 10000 10000

My question is that if cutoff is 10.2 Angestrom, why two CNTs attract each other if the distance between them is below 7.8 Angestrom.

check your LAMMPS output. what does it say about the reason it stops
the minimization?

you are giving a rather large force tolerance. the minimizer will stop
as soon as the forces become smaller than the given threshold and they
*will* be small near the cutoff.

a better way to study whether there is an attraction is to compute the
net force on a CNT via compute/reduce sum
take one of the CNTs away and it should become close to zero.

axel.