IBM closed its $2.1 billion acquisition of Rational Software Corp. Friday, and has begun integrating Rational into its software division.IBM\u00a0closed its $2.1 billion\u00a0acquisition\u00a0of Rational Software Corp. Friday, and has begun integrating Rational into its software division.Rational becomes the fifth unit in IBM's software group, joining Lotus, WebSphere, Tivoli and DB2. While IBM is based in Armonk, N.Y., Rational will continue to operate primarily out of its established Lexington, Mass., location.Mike Devlin, previously Rational's CEO, will continue to lead the organization as general manager of the division, reporting to IBM Software head Steve Mills.Rational makes software tools and services for developing business applications. No major changes in its product line are forthcoming, according to Eric Schurr, who served as Rational's vice president of marketing and now holds that position in the Rational division at IBM."We are always at work on new products and new development," he said. "The color of those things is just a little bluer now than it was before."