Comment on "Density-matrix renormalization-group method for excited states"

In a recent paper [Phys. Rev. B 59, 9699 (1999)], Chandross and Hicks claim to present a new density-matrix renormalization group (DMRG) method for dealing with excited-states of quantum lattice models. The proposed improvement to the DMRG-the inclusion of excited-state wave functions in addition to the ground state in the density matrix when calculating excitations-is in fact standard practice, is clearly stated in White's original papers, and has been used repeatedly by many groups to study excited slates. The authors apply the method to the extended, dimerized Hubbard model for conjugated polymers. The criteria for determining whether states are bound or not are assessed. The authors claim that their results show that the optically important "1B(u)" state is hound (excitonic), in contrast to a previous study. However, the discussion is qualitative, and the authors arrive at conclusions on the basis of results for one lattice size only. We show that when the criterion of Chandross and Hicks is developed into a quantitative definition of particle-hole separation, with the finite-size dependence analyzed, the implication is that the 1B(u) state is unbound, at least in the sense of the density-density correlation function, in keeping with the conclusions of a previous study. [S0163-1829(00)01343-6].