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ctseg

Free plan LinuxMacSelf-hosted

What ctseg is

ctseg is a free TRIQS-based hybridization-expansion segment-picture solver for quantum impurities coupled to an external bath. It supports possibly retarded density-density and spin-spin interactions on the impurity. Under its supported interaction restrictions, the project describes better performance than the generic CTHYB solver. Its double-expansion method samples powers of the hybridization function and perpendicular spin-spin interaction to measure observables. Documented measurements include imaginary-time Green’s functions, self-energy estimators, densities, density correlations, two-particle Green’s functions, state histograms, and perturbation-order histograms. Perpendicular spin-spin expansion works only with a single orbital; multiple orbitals can use hybridization-function expansion only. With perpendicular spin-spin interactions, the improved self-energy estimator requires rotationally invariant interactions. ctseg supports Linux, macOS, and self-hosted offline use. Installation options include Ubuntu 24.04 packages, Linux and OSX Anaconda packages, Docker, and source builds. Source builds require TRIQS with matching major and minor versions.

Who it suits

ctseg is for researchers running quantum-impurity calculations who need a TRIQS-based solver and documented Green’s-function, density, or histogram measurements. It suits Linux, macOS, or self-hosted environments.

What it does well

Free software with offline access
Supports retarded density-density interactions
Measures multiple Green’s-function observables
Docker and package installation options
Self-hosted deployment is available

What to know first

Spin-spin expansion supports one orbital only
Multiple orbitals restrict expansion choices
Source builds require matching TRIQS versions

Plans

Plan	Billed and limits	Price
CTSEG	Free software; requires TRIQS; perpendicular spin-spin expansion is supported only for a single orbital	Free

As triqs.github.io lists them, read 11 Oct 2026. The maker's own page has the final word.

Verdict

ctseg provides a focused solver for supported quantum-impurity interaction models and measurements. Its single-orbital spin-spin restriction is important when planning multi-orbital calculations.

Facts from ctseg's own pages

Algorithm: The double expansion samples powers of the hybridization function and perpendicular spin-spin interaction to measure observables such as the Green’s function. · triqs.github.io, 11 Oct 2026
Authors: The TRIQS/ctseg solver was written by N. Kavokine and O. Parcollet, with contributions from H. Lu and N. Wentzell. · triqs.github.io, 11 Oct 2026
Delivery mode: desktop · triqs.github.io, 24 Sept 2026
Dependency: Building CTSEG from source requires the TRIQS library, and CTSEG must match TRIQS in its major and minor version numbers. · triqs.github.io, 11 Oct 2026
Free plan: Yes · triqs.github.io, 24 Sept 2026
Installation: The project documents Ubuntu 24.04 Debian packages, Linux and OSX Anaconda packages, Docker, and building from source. · triqs.github.io, 11 Oct 2026
Interactions: It supports possibly retarded density-density and spin-spin interactions on the impurity. · triqs.github.io, 11 Oct 2026
License: TRIQS, the software ecosystem CTSEG is based on, is free software distributed under the GPL license. · triqs.github.io, 11 Oct 2026
Measurement constraint: With perpendicular spin-spin interactions, the improved self-energy estimator is available only when the spin-spin interactions are rotationally invariant. · triqs.github.io, 11 Oct 2026
Measurements: Documented observables include imaginary-time Green’s functions, self-energy estimators, densities, density correlations, two-particle Green’s functions, state histograms, and perturbation-order histograms. · triqs.github.io, 11 Oct 2026
Offline access: Yes · triqs.github.io, 24 Sept 2026
Orbital limit: The perpendicular spin-spin expansion is supported only for a single orbital; for multiple orbitals, only expansion in the hybridization function is possible. · triqs.github.io, 11 Oct 2026
Performance: Under its supported interaction restrictions, CTSEG provides better performance than the generic CTHYB solver. · triqs.github.io, 11 Oct 2026
Purpose: CTSEG is a TRIQS-based hybridization-expansion segment-picture solver for quantum impurities coupled to an external bath. · triqs.github.io, 11 Oct 2026
Reproducibility: The installation guide recommends stable releases of TRIQS and its applications for reproducible scientific calculations. · triqs.github.io, 11 Oct 2026
Support: The TRIQS project points users to GitHub Discussions and GitHub issues for help with problems. · triqs.github.io, 11 Oct 2026

Where ctseg stands on Freedom251

Physics Simulation Software#52 of 89