Validity and Limits of Low Order Hybridization Expansion Approaches for Multi-Orbital Systems

• URL: http://arxiv.org/abs/2605.02228v1
• Date: Mon, 04 May 2026 05:01:04 GMT
• Title: Validity and Limits of Low Order Hybridization Expansion Approaches for Multi-Orbital Systems
• Abstract summary: Low-order hybridization expansion methods are widely used in impurity solvers.<n>But their accuracy in genuine multi-orbital settings remains poorly understood.<n>We identify the diagrammatic mechanisms responsible for the breakdown of low-order methods in multi-orbital settings.
• Score: 5.418098386124704
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: Low-order hybridization expansion methods such as the non-crossing approximation (NCA) and the one-crossing approximation (OCA) are widely used impurity solvers in the study of strongly correlated systems, yet their accuracy in genuine multi-orbital settings remains poorly understood. Using the decoupled orbital limit as a controlled reference point, we derive analytic results connecting multi-orbital restricted propagators and Green's functions to their single-orbital counterparts, identify the diagrammatic mechanisms responsible for the breakdown of low-order methods in multi-orbital settings, and determine their regimes of applicability. Our central finding is that the accuracy of these methods is governed by the least correlated orbital: i.e., the orbital with the most rapidly decaying retarded Green's function. That orbital's properties are transferred to all other orbitals through a spurious coupling generated by the truncated expansion, thereby suppressing correlation-induced features such as the Kondo resonance. This occurs even in orbitals that are themselves strongly correlated within single-orbital calculations using the same approximation scheme. We confirm this numerically across representative two-orbital model systems in the steady-state, systematically identifying the parameter regimes in which low-order methods succeed or fail. Our results provide a practical guide for assessing when insights from single-orbital calculations carry over to multi-orbital settings, and serve as a benchmark for the development and validation of higher-order multi-orbital impurity solvers.

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