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 dump.xyz
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
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.
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).
