airebo switch functions not exist

Hello all users,

In the Documentation, it says “the E_LJ term in AIREBO contains a series of switching functions so that the short-ranged LJ repulsion (1/r^12) does not interfere with the energetics captured by the E_REBO term.”
To test that, we ran the following code to give the potential between two carbon at,

units metal

atom_style atomic

boundary p p p

read_data data.2c #two C

pair_style airebo 3.0 1 1

pair_coeff * * CH.airebo C

group left id 1

group right id 2

timestep 0.001

fix 1 right move linear 10 0 0 units box

fix 2 left move linear 0 0 0 units box

thermo_style custom step temp etotal pxx pyy

thermo 1

dump 1 all xyz 1

run 1000

However, test result (see attachd fig) shows there is no switching function to link the LJ repulsion (1/r^12) & the E_REBO term (with a distance in range [2,3]Å), instead what we saw is only a straight line.

Can anybody explain what happens?

Yang Yang
Materials Science department
Rice University


There is no REBO potential from 2 A and beyond, what is the LJ going
to interfere with beyond 2 A?

Please have another look at the 2000 Stuart JCP paper.



first, you do not see LJ repulsion here, but only LJ attraction (1/r^6). LJ repulsion is good for the description of short-ranged interaction when atoms do not form bonds between each other (like in noble gases). REBO describes
short-ranged interactions in covalent-bonded materials with "bonds-related" terms, and it would be physically incorrect to include LJ repulsion in short-range interactions, too. Attractive LJ part in this case mainly describes interaction between molecules.


02.11.2013, 00:48, "杨旸" <[email protected]>:

We do not expert LJ repulsion term when distance between two carbon atoms is less than 3Å, or REBO term if the distance is larger than 2Å. The problem is, there should be some switch function that smoothly link the long-range and the short-range interaction (as mentioned in the original paper), which also reflects the reaction barrier, otherwise I do not see how can AIREBO be used to simulate chemical reactions (as the name R(eactive)EBO indicates).



Please re-read the AIREBO paper more carefully, which is better than
speculating based on limited knowledge of how it should work.

Also like I said, REBO is cut-offed after 2 A for CC pairs, how is LJ
going to interfere with REBO if it is not present?