DMRG truncation: ε_trunc = Σ_{k>χ} λ_k² (sum of discarded Schmidt weights)

White 1992 DMRG / Schollwöck 2005 MPS view: at every bond of the MPS, the Schmidt decomposition |ψ⟩ = Σ_k λ_k |L_k⟩|R_k⟩ (Σ_k λ_k² = 1) determines the optimal bond-dimension-χ truncation — retain the χ largest λ_k. Discarded weight…