Per one of the authors: "As for the nonsymmetry coefficient, this integral is just a measure of the mirror symmetry of fcc around taumax. It would be zero if perfectly symmetric. The integration should really be from 0 to +infinity, but as before, outside of some range of tau, both fcc(taumax-tau) and fcc(taumax+tau) (these are functional evaluations, and not multiplications) will both go to zero. So the integral may be appropriately truncated."
So you were correct and I thank you for pointing this out. As explained, this makes perfect sense. Thanks again, Don