Hello,

I am experimenting with the Tersoff potential in lammps in the case of 2-element systems. Considering only two atoms of different type, and only the repulsive term of the Tersoff potential, without even the distance dependence (\lambda_1=0), it seems to me that according to the formula given in the lammps manual (pair_style tersoff command — LAMMPS documentation), the total energy should be E=\frac{1}{2}\sum_i\sum_{j\neq i} V_{ij}=(A_{12}+A_{21})/2.

However, when I do this calculation with lammps (version 24 Dec 2020), I do obtain either A_{21} or A_{12} as the total energy of the 2-atom system, depending on whether the atom of type 1 or 2 is listed first, respectively.

I do understand that having pair parameters that are not symmetrical may not be reasonable. However, the manual seems to indicate otherwise, it states: “Note that the twobody parameters in entries such as SiCC and CSiSi are often the same, due to the common use of symmetric mixing rules, but this is not always the case.”

Here is the lammps script I am using:

```
units metal
region mybox block -10 10 -10 10 -10 10
create_box 2 mybox
create_atoms 1 single -0.5 0 0
create_atoms 2 single 0.5 0 0
mass 1 1
mass 2 1
pair_style tersoff
pair_coeff * * test.tersoff C Si
run 0
```

and the test.tersoff potential file:

```
C C C 1 0 0 0 0 0 0 0 0 0 2 0.1 0 0
C C Si 1 0 0 0 0 0 0 0 0 0 2 0.1 0 0
C Si C 1 0 0 0 0 0 0 0 0 0 2 0.1 0 0
C Si Si 1 0 0 0 0 0 0 0 0 0 2 0.1 0 1
Si C C 1 0 0 0 0 0 0 0 0 0 2 0.1 0 0
Si C Si 1 0 0 0 0 0 0 0 0 0 2 0.1 0 0
Si Si C 1 0 0 0 0 0 0 0 0 0 2 0.1 0 0
Si Si Si 1 0 0 0 0 0 0 0 0 0 2 0.1 0 0
```

The relevant entries are “C Si Si” and “Si C C”, and in particular the last number which corresponds to the A prefactor, which is set here asymmetrically: A_{CSi}=1 and A_{SiC}=0.

If you run the script above, the total energy is 0, but when you create the atom of type 2 first, the total energy becomes 1.

Can someone please advise me on this subject? Is there a problem with my script, or the way I use the Tersoff formula? Or does the manual mean something different when mentioning SiCC and CSiSi pair parameters that are not necessarily the same? Sorry, I am rather new to this, and I may have missed something in the documentation.