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The cause of your difficulty is trying to do too much with too little understanding. The two potentials that you are trying to use were published by Tersoff in two separate papers (given at the top of each file), each with a different intended purpose. If you read his papers, you will find that in neither case was the potential fit to surface structures, such as the one that you have constructed. I suggest you first verify that the potentials are defined correctly within your LAMMPS simulation by reproducing some simple properties from Tersoff’s original papers, such as lattice constants and elastic constants.
If you really intend to embark on a serious study of silicon surfaces, you should also do more reading on the large body of literature that exists describing surface reconstructions (experimental measurements, calculations with classical potentials, calculations with quantum methods) in Si.
Aidan