Excerpt: The composite interest rate, under Notice 2004-34 released Monday by the Treasury Department, will be based on three long-term corporate indices: Citigroup High Grade Credit Index; the Merrill Lynch Corporates AA-AAA; Lehman Brothers US A Long Credit. That will produce an interest rate assumption of 6.55% for the plan year beginning Jan. 1, 2004, a Treasury Department spokeswoman said.