Open Wilson chains for quantum impurity models: Keeping track of all bath modes

When constructing a Wilson chain to represent a quantum impurity model, the effect of truncated bath modes is neglected. We show that their influence can be kept track of systematically by constructing an ``open Wilson chain'' in which each site is coupled to a separate effective bath of its own. This strategy enables us to cure the so-called mass-flow problem that can arise when using standard Wilson chains to treat impurity models with asymmetric bath spectral functions at finite temperature. We demonstrate this for the strongly sub-Ohmic spin-boson model at quantum criticality where we directly observe the flow towards a Gaussian critical fixed point.