Section 6166(f) provides:
(f) Time for payment of interest
If the time for payment of any amount of tax has been extended under this section-
(1) Interest for first 5 years
Interest payable under section 6601 of any unpaid portion of such amount attributable to the first 5 years after the date prescribed by section 6151(a) for payment of the tax shall be paid annually
(2) Interest for periods after first 5 years
Interest payable under section 6601 on any unpaid portion of such amount attributable to any period after the 5-year period referred to in paragraph (1) shall be paid annually at the same time as, and as a part of, each installment payment of the tax.
(3) Interest in the case of certain deficiencies
In the case of a deficiency to which subsection (e) applies which is assessed after the close of the 5-year period referred to in paragraph (1), interest attributable to such 5-year period, and interest assigned under paragraph (2) to any installment the date for payment of which has arrived on or before the date of the assessment of the deficiency, shall be paid upon notice and demand from the Secretary.
(4) Selection of shorter period
If the executor has selected a period shorter than 5 years under subsection (a)(3), such shorter period shall be substituted for 5 years in paragraphs (1), (2), and (3) of this subsection.
Section 6601(j) provides:
(j) 2-percent rate on certain portion of estate tax extended under section 6166
(1) In general
If the time for payment of an amount of tax imposed by chapter 11 is extended as provided in section 6166, then in lieu of the annual rate provided by subsection (a)-
(A) interest on the 2-percent portion of such amount shall be paid at the rate of 2 percent, and
(B) interest on so much of such amount as exceeds the 2-percent portion shall be paid at a rate equal to 45 percent of the annual rate provided by subsection (a).
For purposes of this subsection, the amount of any deficiency which is prorated to installments payable under section 6166 shall be treated as an amount of tax payable in installments under such section.
(2) 2-percent portion
For purposes of this subsection, the term "2-percent portion" means the lesser of-
(A)(i) the amount of the tentative tax which would be determined under the rate schedule set forth in section 2001(c) if the amount with respect to which such tentative tax is to be computed were the sum of $1,000,000 and the applicable exclusion amount in effect under section 2010(c), reduced by
(ii) the applicable credit amount in effect under section 2010(c), or
(B) the amount of the tax imposed by chapter 11 which is extended as provided in section 6166.
(3) Inflation adjustment
In the case of estates of decedents dying in a calendar year after 1998, the $1,000,000 amount contained in paragraph (2)(A) shall be increased by an amount equal to-
(A) $1,000,000, multiplied by
(B) the cost-of-living adjustment determined under section 1(f)(3) for such calendar year by substituting "calendar year 1997" for "calendar year 1992" in subparagraph (B) thereof.
If any amount as adjusted under the preceding sentence is not a multiple of $10,000, such amount shall be rounded to the next lowest multiple of $10,000.
(4) Treatment of payments
If the amount of tax imposed by chapter 11 which is extended as provided in section 6166 exceeds the 2-percent portion, any payment of a portion of such amount shall, for purposes of computing interest for periods after such payment, be treated as reducing the 2-percent portion by an amount which bears the same ratio to the amount of such payment as the amount of the 2-percent portion (determined without regard to this paragraph) bears to the amount of the tax which is extended as provided in section 6166.