Analysis of electron-correlation effects in strongly correlated systems (N2 and N2+) by applying the density-matrix renormalization-group method and quantum information theory

The dissociation of N-2 and N-2(+) has been studied by using the ab initio density-matrix renormalization-group (DMRG) method. Accurate potential energy surfaces (PESs) have been obtained for the electronic ground states of N-2 (X-1 Sigma(+)(g)) and N-2(+) ( X-2 Sigma(+)(g)) as well as for the N-2(+) excited state B-2 Sigma(+)(u). Inherent 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 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 multireference character featured by these systems.