1. A. F. Bunkin, S. M. Pershin, and A. A. Nurmatov, Phys. Usp. 49, 855 (2006).
2. A. Farkas, Orthohydrogen, Parahydrogen and Heavy Hydrogen (Cambridge Univ., London, 1935), p. 215.
3. H. H. Limbuch, G. Buntkowsky, J. Matthes, S. Grundemann, T. Pery, B. Walaszek, and B. Chaudret, Chem. Phys. Chem. 7, 551 (2006).
4. V. I. Tikhonov and A. A. Volkov, Science 296, 2250 (2002).
5. L. D. Landau and E. M. Lifshitz, Quantum Mechanics, 3rd ed. (Pergamon, Oxford, 1981).