50 USC 2121 - Voluntary contributions

(a) Authority for voluntary contributions 

(1) In general 
Under such regulations as may be prescribed by the Director, a participant may voluntarily contribute additional sums in multiples of one percent of the participants basic pay, but not in excess of 10 percent of such basic pay.
(2) Interest 
The voluntary contribution account in each case is the sum of unrefunded contributions, plus interest
(A) for periods before January 1, 1985, at 3 percent a year; and

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(B) for periods on or after January 1, 1985, at the rate computed under section 8334 (e) of title 5,

compounded annually to the date of election under subsection (b) of this section or the date of payment under subsection (d) of this section.

(b) Treatment of voluntary contributions 
Effective on the date of retirement and at the election of the participant, the participants account shall be
(1) returned in a lump sum;
(2) used to purchase an additional life annuity;
(3) used to purchase an additional life annuity for the participant and to provide for a cash payment on the participants death to a beneficiary; or
(4) used to purchase an additional life annuity for the participant and a life annuity commencing on the participants death payable to a beneficiary, with a guaranteed return to the beneficiary or the beneficiarys legal representative of an amount equal to the cash payment referred to in paragraph (3).

In the case of a benefit provided under paragraph (3) or (4), the participant shall notify the Director in writing of the name of the beneficiary of the cash payment or life annuity to be paid upon the participants death.

(c) Value of benefits 
The benefits provided by subsection (b)(2), (3), or (4) of this section shall be actuarially equivalent in value to the payment provided for in subsection (b)(1) of this section and shall be calculated upon such tables of mortality as may be from time to time prescribed for this purpose by the Director.
(d) Lump-sum payment 
A voluntary contribution account shall be paid in a lump sum at such time as the participant dies or separates from the Agency without entitlement to an annuity. In the case of death, the account shall be paid in the order of precedence specified in section 2071 (c) of this title.
(e) Benefits in addition to other benefits 
Any benefit payable to a participant or to the participants beneficiary with respect to the additional contributions provided under this section shall be in addition to benefits otherwise provided under this subchapter.