Fund (1) Establishment There is established in the Treasury of the United States a fund which shall be known as the Tax Court Judicial Officers' Retirement Fund . The Fund is appropriated for the payment of annuities, refunds, and other payments under this section. (2) Investment of Fund The Secretary shall invest, in interest-bearing securities of the United States, such currently available portions of the Tax Court Judicial Officers’ Retirement Fund as are not immediately required for payments from the Fund. The income derived from these investments constitutes a part of the Fund. (3) Unfunded liability (A) In general Not later than the close of each fiscal year, there shall be deposited in the Tax Court Judicial Officers’ Retirement Fund amounts required to reduce to zero the unfunded liability, if any, of such Fund. (B) Unfunded liability For purposes of subparagraph (A), the term unfunded liability means the amount estimated by the Secretary to be equal to the excess (as of the close of the fiscal year involved) of— (i) the present value of all benefits payable from the Tax Court Judicial Officers’ Retirement Fund, over (ii) the sum of— (I) the present value of future deductions to be withheld under this section from the basic pay of magistrate judges of the Tax Court, plus (II) the balance in such Fund as of the close of such fiscal year. (d) Participation in Thrift Savings Plan (1) Election to contribute A magistrate judge of the Tax Court may elect to contribute out of such individual’s basic pay to the Thrift Savings Fund established by section 8437 of title 5, United States Code. (2) Applicability of title 5 provisions Except as otherwise provided in this subsection, the provisions of subchapters III and VII of chapter 84 of such title 5 shall apply with respect to a magistrate judge of the Tax Court who makes an election under paragraph (1). (3) Special rules (A) Amount contributed The amount contributed by a magistrate judge of the Tax Court to the Thrift Savings Plan in any pay period shall not