Quantum impurity solver based on truncated ED (RASCI) wave function expansion

Quantum impurity models appear in many applications, including nanoscience and the dynamical mean field approximation (DMFT). Many physically relevant impurity models are too large to be solved by exact diagonalization (ED), lack the interaction and hybridization structure required for quantum Monte Carlo (QMC) simulations, or suffer from a severe sign problem. We present an alternative impurity solver inspired by configuration interaction (RASCI) techniques of quantum chemistry and based on a controlled truncation of a wave function expansion. The method can access larger impurity models (impurities with 5 $d$-orbitals and 20 bath orbitals can be easily calculated on a single processor) than can ED and avoids the sign problems of QMC methods. The performance is demonstrated for a cluster DMFT approximation to the two dimensional Hubbard model and for the problem of a Co adatom on a Cu(111) surface.