22 C.F.R. § 19.13-1
"Lump-sum credit" is the compulsory and special contributions to a participant's or former participant's credit in the Fund for his/her first 35 years of service plus interest thereon computed from the midpoint of each service period and compounded at four percent annually to the date of separation or December 31, 1976, whichever is earlier, and after such date, for a participant who separates from the Service after completing at least one year of civilian service and before completing 5 years of such service, at the rate of three percent annually to the date of separation. Interest shall not be paid for a fractional part of a month in the total service or on compulsory and special contributions from the annuitant for recall service or other service performed after the date of separation which forms the basis for annuity.
22 C.F.R. § 19.13-1