Incorrect args for pair coefficients for hybrid potential

LAMMPS LAMMPS Beginners
1

Hello, everyone. Although I set a hybrid potential according to the manuscript, I got an error " Incorrect args for pair coefficients" for "pair_coeff 1 1 airebo CH.airebo C NULL"when I set pair potentials by pair_style hybrid. Then, I substituded “pair_coeff 1 1 airebo CH.airebo C NULL” by “pair_coeff * * airebo CH.airebo C NULL”, the objective of the error was changed to “pair_coeff 1 2 tersoff BNC.tersoff C N”. What should I do? There are two types of atoms in the input files, which are “C” and “N”, respectively. I would appreciate it if I could get your help.
The script is the following.
#model

units metal

dimension 3

variable time_step equal 0.001

timestep ${time_step}

atom_modify map yes

read_data “‘L4_L5_xyza.data’”

change_box all boundary p p p

pair_style hybrid airebo 3.0 tersoff

pair_coeff 1 1 airebo CH.airebo C NULL

pair_coeff 1 2 tersoff BNC.tersoff C N

2

I substitude "pair_coeff 1 1 airebo CH.airebo C NULL” and
“pair_coeff 1 2 tersoff BNC.tersoff C N” by “pair_coeff * * airebo CH.airebo C NULL” and
“pair_coeff * * tersoff BNC.tersoff C N”, respectively. It tells an error “Pair hybrid sub-style is not used”. What should I do?

3

You need to pay better attention to the documentation. Most of what you are doing is just random changes that are in violation of the documented requirements.

For both, tersoff and airebo, the documented requirements are:

  • you must use pair_coeff * *
  • the mappings after the potential file must be exactly one for each atom type in your system

In all of the different variants of command lines that you showed you violated one or both of these.

4

Thanks for your help. I use “Pair_style hybrid/overlay” to solve this problem. However, I need to modify “‘BCN.tersoff’” to turn off ‘C-C-C’.

5

This would be a broken model, too. You must not have the same atoms represented by two different manybody potentials. That would produce inconsistent and thus bogus results.

6

Dear Akohlmey,
I would like to use airebo and tersoff potential to simulate the interaction of C-C and C-N, respectively. How to configure the potential function? There are two types of atoms in the input files, which are “C” and “N”. I will appreciate it if you could give me some suggestions?

7

You can only do this, if you have two separate objects. In that case you need to have two types of carbon atoms, one for tersoff and one for AIREBO. Plus you must have a suitable pairwise additive potential to represent the interactions between the two objects.

Even then it is often better to simulate everything with the same pair style. Hybrid potential setups are always only the second best option.

This all has been discussed many, many times before, so please see the archives for more detailed discussions and explanations.

The fact that something is possible in LAMMPS does not automatically make it a good idea.

8

Thanks for your generous help. I would like to superimpose airebo and tersoff on the C atom, so that I can represent the interaction of “C-C” and “C-N”, respectively. Is it feasible if I use the following script to reach the goal?
pair_style hybrid/overlay airebo 3 tersoff
pair_coeff * * airebo CH.airebo C NULL
pair_coeff * * tersoff BNC.tersoff C N

9

I already explained that this is a broken model producing bogus results. Why would you want to use that?

10

I’m sorry for doing that. I’ve given up this idea and tried to find the right way to do it.