Chebyshev approach to quantum systems coupled to a bath

We propose an alternative concept for the dynamics of a quantum bath, the Chebyshev space, and a method based on this concept, the Chebyshev space method. The Chebyshev space is an abstract vector space that exactly represents the fermionic or bosonic bath degrees of freedom, without a discretization of the bath density of states. Relying on Chebyshev expansions, the Chebyshev space representation of a bath has very favorable properties with respect to extremely precise and efficient calculations of ground state properties, static and dynamical correlations, and time evolution for a great variety of quantum systems. The aim of the present work is to introduce the Chebyshev space in detail and to demonstrate the capabilities of the Chebyshev space method. Although the central idea is derived in full generality, the focus is on model systems coupled to fermionic baths. In particular, we address quantum impurity problems, such as an impurity in a host or a bosonic impurity with a static barrier, and the motion of a wave packet on a chain coupled to leads. For the bosonic impurity, the phase transition from a delocalized electron to a localized polaron in arbitrary dimension is detected. For the wave packet on a chain, we show how the Chebyshev space method implements different boundary conditions, including transparent boundary conditions replacing infinite leads. Furthermore, the self-consistent solution of the Holstein model in infinite dimension is calculated. With the examples, we demonstrate how highly accurate results for system energies, correlation and spectral functions, and time dependence of observables are obtained with modest computational effort.