  The dissociation of $\rm N_2$ and $\rm N_2^+$ has been studied by using the
\emph{ab initio} Density Matrix Renormalization Group (DMRG) method. Accurate
Potential Energy Surfaces (PES) have been obtained for the electronic ground
states of $\rm N_2$ ($\rm X^1\Sigma_g^+$) and $\rm N_2^+$ ($\rm X^2\Sigma_g^+$)
as well as for the $\rm N_2^+$ excited state $\rm B^2\Sigma_u^+$. Inherently to
the DMRG approach, the eigenvalues of the reduced density matrix ($\rho$) and
their correlation functions are at hand. Thus we can apply Quantum Information
Theory (QIT) directly, and investigate how the wave function changes along the
PES and depict differences between the different states. Moreover by
characterizing quantum entanglement between different pairs of orbitals and
analyzing the reduced density matrix, we achieved a better understanding of the
multi-reference character featured by these systems.
