T. V. Tscherbul, P. Zhang, H. R. Sadeghpour, A. Dalgarno, N. Brahms, Y. S. Au, and J. M. Doyle Harvard-MIT Center for Ultracold Atoms, Cambridge, Massachusetts 02138, USA Institute for Theoretical Atomic, Molecular, and Optical Physics, Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA Received 3 October 2008; published 31 December 2008