Dresden 2026 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 63: Quantum Dynamics and Many-body Systems – Poster (joint session DY/TT)

TT 63.3: Poster

Wednesday, March 11, 2026, 15:00–18:00, P5

Nonequilibrium Green Function Simulations for Large Systems — •Erik Schroedter, Jan-Philip Joost, and Michael Bonitz — Christian-Albrechts-Universität zu Kiel, Kiel, Germany

Nonequilibrium Green Functions (NEGF) provide a powerful framework for accurately simulating the dynamics of correlated many-body systems. A major limitation of standard NEGF approaches is the cubic scaling of computational cost with the number of time steps. Recently, the G1-G2 scheme [1] overcame this limitation, achieving linear scaling. However, it introduces its own challenges, such as numerical instabilities at strong coupling and large memory requirements, which have so far restricted simulations to small systems with fewer than 150 basis states. Here, we introduce a NEGF-based quantum fluctuations approach (NEGF-QF)[2] that builds on earlier works [3] to efficiently factorize the two-particle Green function. This method drastically reduces computational costs for advanced self-energy approximations, including GW and T-matrix, while enabling straightforward parallelization. As a result, NEGF-QF allows simulations of systems with up to ten thousand basis states. We demonstrate the approach for large Hubbard clusters and graphene nanoribbons, illustrating its effectiveness for large, strongly correlated systems.

This work was funded by the Deutsche Forschungsgemeinschaft (DFG), Project No. 464370560.

[1] Schlünzen et al., Phys. Rev. Lett. 124, 076601 (2020)

[2] Schroedter et al., to be published (2026)

[3] Schroedter et al., Cond. Matt. Phys. 25, 23401 (2022)

Keywords: nonequilibrium Green functions; quantum fluctuations; stochastic methods; Hubbard clusters; graphene nanoribbons