When ∆ i = 0, the integrand is a constant function and T i → ∞. A possibility to avoid such divergence is to change the upper limit of the integral to a value that is directly proportional to the energy difference ∆ i , such that when ∆ i = 0 and the integrand is constant, the integral is bounded by a finite upper limit. This strategy is implemented by the following expression, where, to accommodate for the possibility for negative denominators, we have introduced the absolute value and sign functions at appropriate places: