NRG Ljubljana is an open-source framework for numerical renormalization group (NRG) calculations of quantum impurity problems. It supports models such as the Kondo exchange model and the single-impurity Anderson model, together with multi-impurity and multichannel generalizations.
The framework computes thermodynamic quantities, including magnetic and charge susceptibilities, entropy, and heat capacity; expectation values of local operators; spectral functions; and transport properties. Its annotated output supports inspection, post-processing, and plotting.
Model preparation
New models are prepared in Mathematica using the SNEG library for symbolic calculations with second-quantized operators. Hamiltonians and observables can be written in familiar second-quantization notation. The initialization stage constructs the basis and operator matrices and prepares the input for the NRG iteration.
Calculations with supplied inputs can run without Mathematica. Reusable model templates also allow parameter sweeps without repeating the Mathematica initialization step for each calculation. See the single-impurity Anderson-model tutorial for a practical starting point.
Numerical calculations
The NRG iteration is implemented in C++, using numerical linear-algebra libraries for diagonalization and matrix operations. Current releases require C++20 and support parallel CPU calculations. Version 2026.09 adds optional NVIDIA CUDA acceleration for diagonalization and selected operator-recalculation matrix products in CUDA-enabled source builds.
For installation, model setup, numerical parameters, and code internals, see the documentation. Source code is available on GitHub; older distributions and teaching material are in the legacy archive.