A simple impurity solver is shown to capture impurity-induced superconducting subgap states in quantitative agreement with the numerical renormalization group and quantum Monte Carlo simulations. The solver is based on the exact diagonalization of a single-impurity Anderson model with discretized superconducting reservoirs including only a small number of effective levels. Their energies and couplings to the impurity $d$ level are chosen so as to best reproduce the Matsubara frequency dependence of the hybridization function. We provide a number of critical benchmarks and demonstrate the solver's efficiency in combination with the reduced basis method [V. V. Baran and D. R. Nichita, Phys. Rev. B 107, 144503 (2023)] by calculating the phase diagram for an interacting three-terminal junction.

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• Impurities in superconductors
• Superconducting gap
• Tunnel junctions
• Josephson junctions
• Quantum dots
• Superconducting devices
• Approximation methods for many-body systems
• Exact diagonalization
• Methods in superconductivity
• Multichannel Anderson model