[lammps-users] Question about pair_style hybrid/overlay

Hi, Seven

I’ve been trying to start a simulation with a hybrid/overlay of AIREBO and Tersoff.
But I have a question about hybrid/overlay as follows command:

atom 1 is C atom

atom 2 is N atom

pair_style hybrid/overlay tersoff airebo 2.5 0 0
pair_coeff * * tersoff NC.tersoff C N
pair_coeff * * airebo CH.airebo C NULL

Is it means that C atom will use two potentials for any interation between C-N, C-C and N-N?
or tersoff will only use for C-N and airebo will use for C-C ?

Best,

Ph.D. student
Shih-Kai, Chien
Department of Mechanical Engineering
National Cheng-Kung University
Tainan, Taiwan 70701

2010/1/6 簡 士凱 <[email protected]>:

Hi, Seven

I've been trying to start a simulation with a hybrid/overlay of AIREBO and
Tersoff.
But I have a question about hybrid/overlay as follows command:

# atom 1 is C atom
# atom 2 is N atom
pair_style hybrid/overlay tersoff airebo 2.5 0 0
pair_coeff * * tersoff NC.tersoff C N
pair_coeff * * airebo CH.airebo C NULL

this way of mixing potentials is _very_ strange
and doesn't really make much sense to me.

have you tried whether this will run at all (i would guess not)?

Is it means that C atom will use two potentials for any interation between
C-N, C-C and N-N?

hybrid/overlay will add potentials. so if you use pair_coeff * *
it will compute _all_ combinations. also keep in mind that
for example tersoff has 3-body potential terms, i.e. it will
also compute C-C-C, C-N-C, C-C-N, N-N-C, N-N-N, and
N-C-N interactions.

it might be better, if you first explain what you want to achieve
and why you want to choose these potentials in this way.
this way people here can give you more result oriented
suggestions.

cheers,
    axel.

Thanks for your advices, alex

Date: Wed, 6 Jan 2010 13:55:20 -0500
Subject: Re: [lammps-users] Question about pair_style hybrid/overlay
From: [email protected]
To: [email protected]…1188…
CC: [email protected]

2010/1/6 ² �h�� <[email protected]…1188…>:

Hi, Seven

I’ve been trying to start a simulation with a hybrid/overlay of AIREBO and
Tersoff.
But I have a question about hybrid/overlay as follows command:

atom 1 is C atom

atom 2 is N atom

pair_style hybrid/overlay tersoff airebo 2.5 0 0
pair_coeff * * tersoff NC.tersoff C N
pair_coeff * * airebo CH.airebo C NULL

this way of mixing potentials is very strange
and doesn’t really make much sense to me.

have you tried whether this will run at all (i would guess not)?
yes, i have tried above command and it can be run.

Is it means that C atom will use two potentials for any interation between
C-N, C-C and N-N?

hybrid/overlay will add potentials. so if you use pair_coeff * *
it will compute all combinations. also keep in mind that
for example tersoff has 3-body potential terms, i.e. it will
also compute C-C-C, C-N-C, C-C-N, N-N-C, N-N-N, and
N-C-N interactions.
the tersoff use for 3-body, but the pair_coeff tersoff in the manual says that the 1st 2 arguments must be* *.

it might be better, if you first explain what you want to achieve
and why you want to choose these potentials in this way.
this way people here can give you more result oriented
suggestions.
i want the carbon nanotube use airebo potential and some N atom near the carbon nanotube use tersoff potential.
so i choose these potentials in this way. Thanks for your kindly helps in advance.
Best,
ShihKai

yes - your interpretation is correct (except
that the interactions are not all pairwise).
But I agree with Axel that it is a very odd
model. Why would you want a set of
atoms to interact via both Tersoff and AIREBO?

Steve

2010/1/6 簡 士凱 <[email protected]>:

Steve,

thanks for your response. I want the carbon nanotube use airebo potential
and some N atom near the carbon nanotube use tersoff potential. So I choose these potentials in this way.
How should i set the hybrid/overlay commands are much better?
Thanks for your kindly helps in advance.

Best,

Ph.D. student
Shih-Kai, Chien
Department of Mechanical Engineering
National Cheng-Kung University
Tainan, Taiwan 70701

2010/1/7 Chien ShihKai <[email protected]>:

Steve,

thanks for your response. I want the carbon nanotube use airebo potential
and some N atom near the carbon nanotube use tersoff potential. So I choose
these potentials in this way.

if you want to mix and match _pairwise_ interactions between different
pairs of atoms, you don't use hybrid/overlay, but just plain hybrid.

since both airebo and tersoff contain manybody terms, it is not at
all obvious how you distribute those manybody terms for the setup
that you are describing. whatever you do, it is most likely that you
create inconsistencies in the interactions.

strictly speaking those should not be _pair_ potentials. their implementation
somewhat "abuses" the pair style, since the many body terms are short ranged
and thus within the pairwise cutoff and once can use the pair
potential infrastructure.

why not simply use the tersoff potential throughout?

cheers,
    axel.

axel,

Thanks a lot. I will try the tersoff potential throughout.
Because next model i will add H atom in my simulation, so i hope can use the tersoff and airebo.

atom 1 is C atom

atom 2 is N atom

atom 3 is H atom

pair_style hybrid/overlay tersoff airebo 2.5 0 0
pair_coeff * * tersoff NC.tersoff C N NULL
pair_coeff * * airebo CH.airebo C NULL H

but now i found the problem about pair_coeff tersoff, the NULL value seems can’t set.
If i set the NULL value, an error occurs. or my command is wrong?

Best,

ShihKai