Schmidt values, area law vs critical scaling S ~ (c/3)log L
DMRG works by keeping only the χ largest Schmidt values in a bipartition of the chain. The entanglement entropy S = −Σλᵢ²log(λᵢ²) obeys an area law S = const for gapped 1D systems (e.g. Haldane chain), but diverges logarithmically S ≈ (c/3)log(L) at quantum critical points (CFT), where c is the central charge. The truncation error ε = 1 − Σλᵢ² controls DMRG accuracy — smaller Schmidt values can be safely discarded.