Jianxin,
If your period box size is less than the cut-off distance, would your atoms not be seeing themselves in their periodic image? That usually causes all kinds of issues. Also 6 sigma is a big cut off. The usual number I see for airebo is 2.5 to 3. I believe the potential was fit for 3 sigma.
Make your system bigger and see if that fixes the problem.
Dave Schall