Interest rates; underpayments and overpayments. The rates for interest determined under section 6621 of the Code for the calendar quarter beginning April 1, 2011, will be 4 percent for overpayments (3 percent in the case of a corporation), 4 percent for underpayments, and 6 percent for large corporate underpayments. The rate of interest paid on the portion of a corporate overpayment exceeding $10,000 will be 1.5 percent.