26 U.S.C. § 5684

(a) Failure to pay tax Whoever fails to pay any tax imposed by part I of subchapter A at the time prescribed shall, in addition to any other penalty provided in this title, be liable to a penalty of 5 percent of the tax due but unpaid. (b) Applicability of section 6665 The penalties imposed by subsection (a) shall be assessed, collected, and paid in the same manner as taxes, as provided in section 6665(a). (c) Cross references (1) For provisions relating to interest in the case of taxes not paid when due, see section 6601. (2) For penalty for failure to file tax return or pay tax, see section 6651. (3) For additional penalties for failure to pay tax, see section 6653. (4) For penalty for failure to make deposits or for overstatement of deposits, see section 6656. (5) For penalty for attempt to evade or defeat any tax imposed by this title, see section 7201. (6) For penalty for willful failure to file return, supply information, or pay tax, see section 7203.