From my resources page:

  • For the text of the LXX, start with Rahlf's Septuaginta or the Old Testament in Greek by Swete or the editions of it in Bible software or online.
    • A morphologically coded text, which generally comes with most Bible software, is needed. The morphological data in electronic editions appear to trace back to that of CCAT at the Univ. of Pennsylvania or are based on the Kraft/Taylor/Wheeler Morphology Database. (For now, the Logos LXX has known coding errors.)
    • Rahlf and Swete list some variants, but the only real critical edition is the Göttingen Septuagint. (The Göttingen Septuagint is very expensive. Look for it in the library. Logos is in the process of digitizing it. Another critical edition by A. E. Brooke, N. McLean, and H. St-J. Thackeray was never completed.)