Occupation-Space Variational Quantum Optimization
Abstract
Occupation-Space Variational Quantum Optimization (OS-VQO) is introduced as a new framework for quantum electronic structure calculations in which the quantum variational object is an occupation state rather than a many-electron wavefunction. Consequently, the quantum processor operates directly on the occupation numbers, the fundamental variational variables of Natural Orbital Functional Theory (NOFT). The framework is realized through a logarithmically scaling qudit encoding of electron-pair occupation spaces and a hierarchical variational circuit that spans the complete physically admissible occupation manifold while enforcing all N-representability constraints by construction. Noiseless quantum simulations and raw executions on IBM Quantum processors for hydrogen systems up to H216 demonstrate excellent agreement without error mitigation or post-processing, constituting, to the best of our knowledge, the largest raw quantum hardware demonstration reported for a strongly correlated electronic structure calculation. These results demonstrate that quantum variational optimization need not rely on many-electron wavefunctions, opening a new direction for quantum electronic structure calculations based on functional theories.
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BibTeX
@misc{salazar2026occupationspace,
title = {{Occupation-Space Variational Quantum Optimization}},
author = {Edison X. Salazar and Mario Piris},
howpublished = {ChemRxiv},
year = {2026},
month = sep,
doi = {10.26434/chemrxiv.15008226/v1},
url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15008226/v1},
note = {CC BY-NC-ND},
}
RIS
TY - UNPB
TI - Occupation-Space Variational Quantum Optimization
AU - Edison X. Salazar
AU - Mario Piris
PY - 2026
DA - 2026/09/02
PB - ChemRxiv
DO - 10.26434/chemrxiv.15008226/v1
UR - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15008226/v1
AB - Occupation-Space Variational Quantum Optimization (OS-VQO) is introduced as a new framework for quantum electronic structure calculations in which the quantum variational object is an occupation state rather than a many-electron wavefunction. Consequently, the quantum processor operates directly on the occupation numbers, the fundamental variational variables of Natural Orbital Functional Theory (NOFT). The framework is realized through a logarithmically scaling qudit encoding of electron-pair occupation spaces and a hierarchical variational circuit that spans the complete physically admissible occupation manifold while enforcing all N-representability constraints by construction. Noiseless quantum simulations and raw executions on IBM Quantum processors for hydrogen systems up to H216 demonstrate excellent agreement without error mitigation or post-processing, constituting, to the best of our knowledge, the largest raw quantum hardware demonstration reported for a strongly correlated electronic structure calculation. These results demonstrate that quantum variational optimization need not rely on many-electron wavefunctions, opening a new direction for quantum electronic structure calculations based on functional theories.
ER -