### Abstract

We study the concept of valence bond states from the perspective of entanglement theory. This leads to several generalizations with powerful applications for the simulation of quantum spin systems. In particular, we extend the powerful Density Matrix Renormalization Group (DMRG) simulation method to the cases of periodic boundary conditions, finite-T, time-evolution, and 2 dimensions. We also discuss the relationship between entanglement and correlations in those systems, leading to a natural definition of entanglement length.