Comment on "Density-matrix renormalization-group method for excited states" - art. no. 157101

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 de aling with excited-states of quantum lattice models. The proposed improveme nt 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 h as been used repeatedly by many groups to study excited slates. The authors apply the method to the extended, dimerized Hubbard model for conjugated p olymers. The criteria for determining whether states are bound or not are a ssessed. The authors claim that their results show that the optically impor tant "1B(u)" state is hound (excitonic), in contrast to a previous study. H owever, the discussion is qualitative, and the authors arrive at conclusion s 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 definitio n of particle-hole separation, with the finite-size dependence analyzed, th e 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 o f a previous study. [S0163-1829(00)01343-6].