22 U.S.C. § 4043

(a) Career appointees or candidates; chiefs of mission Except as provided in subsection (d), the following members of the Service (hereinafter in this part referred to as “participants”) shall be entitled to the benefits of the System: (1) Every member who is serving under a career appointment or as a career candidate under section 3946 of this title — (A) in the Senior Foreign Service, or (B) assigned to a salary class in the Foreign Service Schedule. (2) Every chief of mission, who is not a participant under paragraph (1), who— (A) has served as chief of mission for an aggregate period of 20 years or more, and (B) has paid into the Fund a special contribution for each year of such service in accordance with section 4045 of this title . (b) Executive branch appointees Any otherwise eligible member of the Service who is appointed to a position in the executive branch by the President, by and with the advice and consent of the Senate, or by the President alone, shall not by virtue of the acceptance of such appointment cease to be eligible to participate in the System. (c) Binational Center Grantees In addition to the individuals who are participants in the System under subsection (a), any individual who was appointed as a Binational Center Grantee and who completed at least 5 years of satisfactory service as such a grantee or under any other appointment under the Foreign Service Act of 1946 may become a participant in the System, and shall receive credit for such service if an appropriate special contribution is made to the Fund in accordance with section 4045(d) or (f) of this title. (d) Exclusion An individual subject to the Foreign Service Pension System (described in part II) is not a participant in this System.