1. C. Cohen-Tannoudji, B. Diu, F. Laloë, Quantum Mechanics, vol. 1, 1st edn. (Wiley, New York, 1978)
2. C. Cohen-Tannoudji, B. Diu, F. Laloë, Quantum Mechanics, vol. 2, 1st edn. (Wiley, New York, 1978)
3. U. D. Jentschura, V. Debierre, C. M. Adhikari, A. Matveev, and N. Kolachevsky, Long-range interactions of excited hydrogen atoms. II. Hyperfine-resolved ( $$2S; 2S$$ 2 S ; 2 S )-system, submitted to Physical Review A (2016)
4. S. Wolfram, The Mathematica Book, 4th edn. (Cambridge University Press, Cambridge, 1999)
5. See the URL http://en.m.wikipedia.org/wiki/ Avoided_crossing