In Revenue Ruling 2014-14 (PDF), IRS releases interest rates for underpayments and overpayments of taxes. For the third quarter of 2014 (July 1 to September 30, 2014), the rates are:
Individual underpayments 3%
Individual overpayments 3%
Corporate large underpayment rate 5%
Corporate underpayment rate 3%
Corporate overpayments > $10,000 0.5%
Corporate overpayments <= $10,000 2%
Underpayment means what the taxpayer pays while overpayment means what IRS pays on refunds.
A large corporate underpayment is an underpayment over $100,000 by a C corporation.