The real natural rate should be in line with the private markets with regards to safe, liquid short term assets. There is an issue when you hit 0% nominal, call

The real natural rate should be in line with the private markets with regards to safe, liquid short term assets. There is an issue when you hit 0% nominal, called the zero lower bound problem (https://en.wikipedia.org/wiki/Zero_lower_bound https://en.wikipedia.org/wiki/Zero_lower_bound). Mechanically you can solve that by keeping inflation high enough that you don't reach negative nominal rates even when you have negative real rates.