5 CFR §831.405

1. (a)Interest on voluntary contributions is computed under § 831.105.
2. (b)Voluntary contributions begin to earn interest on the date deposited by OPM.
3. (c)Except as provided in paragraph (d) of this section, voluntary contributions stop earning interest on the earliest of—
   1. (1)The date when OPM authorizes payment to the individual of the balance as a withdrawal (831.406);
   2. (2)The date when the employee or Member separates or transfers to a position not subject to CSRS or FERS; or
   3. (3)The date when the employee transfers to a retirement system other than CSRS or FERS.
4. (d)If an employee separates with entitlement to a deferred annuity and either dies without withdrawing his or her voluntary contributions or uses his or her voluntary contributions to purchase additional annuity, voluntary contributions stop earning interest on the earlier of—
   1. (1)The date the former employee or Member dies; or
   2. (2)The commencing date of the former employee's or Member's deferred annuity.