The implementation of analytic BOPs and tight-binding in BOPfox is described in the central BOPfox paper. This paper needs to be cited in publications that made use of BOPfox:

BOPfox program for tight-binding and analytic bond-order potential calculations,
T. Hammerschmidt, B. Seiser, M. E. Ford, A.N. Ladines, S. Schreiber, N. Wang, J. Jenke, Y. Lysogorskiy, C. Teijeiro, M. Mrovec, M. Cak, E. R. Margine, D. G. Pettifor,
and R. Drautz, arxiv: 1803.07491.

A recent review of the BOP methodology can be found here:

R. Drautz, T. Hammerschmidt, M. Cak, D. G. Pettifor: Bond-order potentials: Derivation and parameterization for refractory elements, Mod. Sim. Mat. Sci. Eng. 23 (2015) 074004. DOI: 10.1088/0965-0393/23/7/074004