Even when today's rates are positive the black-76 model doesn't price in the possibility that rates will become be negative in the future. For this reason the

Even when today's rates are positive the black-76 model doesn't price in the possibility that rates will become be negative in the future. For this reason the model breaks down well before the rate actually becomes negative, and practitioners have had to worry about this for quite some time! Using a shifted lognormal model is a quick fix, as the article mention. Then you just need to pick a shift size...