On the Spin Symmetry of Fractional-Occupation Mean-Field Approaches

Md Rafi Ul Azam; Rachel Campbell; Kay Carter-Fenk

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arXiv · 2610.38224

On the Spin Symmetry of Fractional-Occupation Mean-Field Approaches

Md Rafi Ul Azam·Rachel Campbell·Kay Carter-Fenk

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Abstract

We identify a spin-symmetry dilemma rooted in the representability problem of fractional-occupation densities, which break spin symmetry even for restricted orbital references. We propose a resolution that leverages complex polarization to restore pure-state N-representability and exact spin eigenstates while retaining improvements to the energy and density from occupation smearing. Our results have implications for widely-used smearing methods in solid-state simulation and for their use as a proxy for static correlation in molecules.

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Md Rafi Ul Azam, Rachel Campbell, Kay Carter-Fenk. 2026-10-28. On the Spin Symmetry of Fractional-Occupation Mean-Field Approaches. https://arxiv.org/abs/2610.38224

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