Generally, in the case of a corporation, the underpayment rate is the federal short-term rate plus 3 percentage points and the overpayment rate is the federal short-term rate plus 2 percentage points. The rate for large corporate underpayments is the federal short-term rate plus 5 percentage points.
Back taxes have a baseline rate that grows at certain increments of lateness but is ultimately capped. I don't think it's in any way tied to the base or prime rate.
Based on what happens in my country, it could be both penalty + some interest rates based on the reference rate (which definitely haven't been zero for the past two years btw) but not equal to it.
> Microsoft owes the Internal Revenue Service (IRS) $28.9 billion in back taxes, not including penalties and interest