Density Matrix Renormalization Group Study of One Dimensional Models Beyond the Born-Oppenheimer Approximation

We study one dimensional models of molecules and solids where both the electrons and nuclei are treated as quantum particles, going beyond the usual Born-Oppenheimer approximation. The continuous system is approximated by a grid which computationally resembles a ladder, with the electrons living on one leg and the nuclei on the other. To simulate DMRG well with this system, a three-site algorithm has been implemented. We also use a compression method to treat the long-range interactions between charged particles. We find that 1D diatomic molecules ("H2") with spin-1/2 nuclei in the spin triplet state will unbind when the mass of the nuclei reduces to only a few times larger than the electron mass. The molecule with nuclei in the singlet state always binds. The case of spin-0 bosonic nuclei is investigated as well.