  The Similarity Renormalization Group reduces the off-shellness by driving the
evolved interaction towards a diagonal band. We analyze the infrared limit and
the corresponding on-shell interactions and its consequences for light nuclei.
Using a harmonic oscillator shell model we obtain a Tjon line B(4He)=
4B(3H)-3B(2H) which can be understood from a combinatorics counting of nucleon
pairs and triplets in the triton and alpha-particle and compares favorably with
realistic calculations.
