https://doi.org/10.1140/epjs/s11734-025-01922-3
Regular Article
A method to derive material-specific model descriptions of spin degrees of freedom coupled to a fermionic bath for materials displaying prevalent spin physics
HQS Quantum Simulations GmbH, Rintheimer Straße 23, 76131, Karlsruhe, Germany
a
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Received:
31
March
2025
Accepted:
5
September
2025
Published online:
9
October
2025
Abstract
Magnetism and spin physics are true quantum mechanical effects and their description usually requires multi-reference methods and is often hidden in the standard description of molecules in quantum chemistry. In this work, we present a twofold approach to the description of spin physics in molecules and solids. First, we present a method that identifies the single-particle basis in which a given subset of the orbitals is equivalent to spin degrees of freedom for models and materials, which feature significant spin physics at low energies. We introduce a metric for the spin-like character of a basis orbital, of which the optimization yields the basis containing the optimum spin-like basis orbitals. Second, we demonstrate an extended Schrieffer–Wolff transformation method to derive the effective Hamiltonian acting on the subspace of the Hilbert space in which the charge degree of freedom of electron densities in the spin-like orbitals is integrated out. The method then yields an effective Hamiltonian describing spins coupled to a fermionic environment. This extended Schrieffer–Wolff transformation is applicable to a wide range of Hamiltonians and has been utilized in this work for model Hamiltonians as well as the Hamiltonian describing the active orbital space of molecular chromium bromide. This is achieved by reformulating the highly non-linear Schrieffer–Wolff equations into a linear set of equations corresponding to an operator basis.
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© The Author(s), under exclusive licence to EDP Sciences, Springer-Verlag GmbH Germany, part of Springer Nature 2025
Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.
