Ground and Excited States from Ensemble Variational Principles

Lexin Ding, Cheng-Lin Hong, and Christian Schilling

Faculty of Physics, Arnold Sommerfeld Centre for Theoretical Physics (ASC), Ludwig-Maximilians-Universität München, Theresienstr. 37, 80333 München, Germany
Munich Center for Quantum Science and Technology (MCQST), Schellingstrasse 4, 80799 München, Germany

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Abstract

The extension of the Rayleigh-Ritz variational principle to ensemble states $\rho_{\mathbf{w}}\equiv\sum_k w_k |\Psi_k\rangle \langle\Psi_k|$ with fixed weights $w_k$ lies ultimately at the heart of several recent methodological developments for targeting excitation energies by variational means. Prominent examples are density and density matrix functional theory, Monte Carlo sampling, state-average complete active space self-consistent field methods and variational quantum eigensolvers. In order to provide a sound basis for all these methods and to improve their current implementations, we prove the validity of the underlying critical hypothesis: Whenever the ensemble energy is well-converged, the same holds true for the ensemble state $\rho_{\mathbf{w}}$ as well as the individual eigenstates $|\Psi_k\rangle$ and eigenenergies $E_k$. To be more specific, we derive linear bounds $d_-\Delta{E}_{\mathbf{w}} \leq \Delta Q \leq d_+ \Delta{E}_{\mathbf{w}}$ on the errors $\Delta Q $ of these sought-after quantities. A subsequent analytical analysis and numerical illustration proves the tightness of our universal inequalities. Our results and particularly the explicit form of $d_{\pm}\equiv d_{\pm}^{(Q)}(\mathbf{w},\mathbf{E})$ provide valuable insights into the optimal choice of the auxiliary weights $w_k$ in practical applications.

Describing the structure and behaviour of quantum systems — from tiny nuclei, atoms via bio-molecules to macroscopically large solid bodies — is one of the central challenges in modern physics, chemistry and materials science. In practise this means to compute the different energy levels of quantum systems in order to understand how light and other external influences may excite and more general affect them. Prominent examples are photovoltaic, phototosynthesis and quantum technologies. The common procedure of determining individual energy levels only successively results inevitably, however, in an unpleasant accumulation of computational errors. To circumvent this deficiency, our work establishes by rigorous means an alternative and conceptually appealing approach which treats all relevant energy levels on an equal footing my describing them as a joint ensemble.

To accomplish this, our work reveals and exploits a fruitful link between ensemble quantum states and geometry: Inspired by the concept of transport polytopes, which plays a crucial role in economy, computer science and mathematics, we elucidate by analytical means the effect of external influences on the quantum system, whose energy contribution is `transported' into the excitation structure. In particular, we conclusively understand how the (weights of the) ensemble needs to be chosen such that the physical properties of the targeted energy levels can be predicted with maximal accuracy.

Accordingly, our work extends the toolbox of quantum scientists by a scheme for effectively targeting the important excited states with potentially higher accuracy with applications not only in classical but also in quantum computing.

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[2] Harper R Grimsley and Francesco A Evangelista, "Challenging excited states from adaptive quantum eigensolvers: subspace expansions vs. state-averaged strategies", Quantum Science and Technology 10 2, 025003 (2025).

[3] Adrien Kahn, Luca Gravina, and Filippo Vicentini, "Variational subspace methods and application to improving variational Monte Carlo dynamics", Quantum 10, 2082 (2026).

[4] Mabel. A. Moreno, Anjana Devi, Luis Velasquez, Guillermo Gonzalez, and Daniel Navas, "Metamorphosis of Multilayered‐(NH4)2V7O16 Squares to Zinc Vanadate and w‐ZnO Nanoparticles via Zinc Metalation through Atomic Layer Deposition: Precision and Sustainability in Material Transformation", Chemistry–Methods 6 1, e202500066 (2026).

[5] Chih-Chun Wang and Christian Schilling, "Geometric approach to strongly correlated bosons: From N -representability to the generalized Bose-Einstein condensate force", Physical Review B 113 22, 224511 (2026).

[6] Michael Filatov(Gulak) and Seung Kyu Min, "Progress toward a formulation of density-functional theory for matrix representations", Physical Review A 111 6, 062818 (2025).

[7] Julia Liebert, Federico Castillo, Jean-Philippe Labbé, Tomasz Maciazek, and Christian Schilling, "Solving one-body ensemble N-representability problems with spin", Quantum 9, 1921 (2025).

[8] Julia Liebert, Anna O Schouten, Irma Avdic, Christian Schilling, and David A Mazziotti, "Refining ensemble N-representability of one-body density matrices from partial information", New Journal of Physics 27 12, 124511 (2025).

[9] Harper R. Grimsley and Francesco A. Evangelista, "Thermal Weight Determination and Interstate Coupling in State-Averaged ADAPT-VQE", Journal of Chemical Theory and Computation 21 24, 12557 (2025).

[10] Francisco M. Fernández, "On the Application of the Rayleigh-Ritz Method to a Projected Hamiltonian", (2024).

[11] Akilan Rajamani, Martin Beseda, Benjamin Lasorne, and Bruno Senjean, "How an Equi-Ensemble Description Systematically Outperforms the Weighted Ensemble Variational Quantum Eigensolver", The Journal of Physical Chemistry A 130 22, 4171 (2026).

[12] María Laura Olivera-Atencio, Jesús Casado-Pascual, and Denis Lacroix, "Exploring fixed points and eigenstates of quantum systems with reinforcement learning", Physical Review Research 8 3, 033087 (2026).

[13] Yuchen Wang, Cameron Cianci, Irma Avdic, Rishab Dutta, Samuel Warren, Brandon Allen, Nam P. Vu, Lea F. Santos, Victor S. Batista, and David A. Mazziotti, "Characterizing Conical Intersections of Nucleobases on Quantum Computers", Journal of Chemical Theory and Computation 21 3, 1213 (2025).

[14] Mabel Moreno, Anjana Devi, David Zanders, Miryam Arredondo, Davide Mariotti, Ruairi McGlynn, Sindy Devis, Simón Guerrero, Eglantina Benavente, Matias Alegría, Yusser Olguin, Lorena Lobos‐Gonzalez, Kevin Guzmán, Elizabeth Rivas‐Yañez, Paula Solar, Valentin Cepus, Michael Krause, and Luis Velásquez, "Atomic Layer Deposition Processes: Versatile Platforms for Engineering ZnO‐Chitosan Biointerfaces", Advanced Healthcare Materials 15 26, e71329 (2026).

[15] Francisco M. Fernández, "On the application of the Rayleigh–Ritz method to a projected Hamiltonian", Journal of Mathematical Chemistry 63 6, 1343 (2025).

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[17] Silvie Illésová, Martin Beseda, Saad Yalouz, Benjamin Lasorne, and Bruno Senjean, "Transformation-Free Generation of a Quasi-Diabatic Representation from the State-Average Orbital-Optimized Variational Quantum Eigensolver", Journal of Chemical Theory and Computation 21 11, 5457 (2025).

[18] Shaojun Wu, Shan Jin, Abolfazl Bayat, and Xiaoting Wang, "Enhancing the reachability of variational quantum algorithms via input-state design", Communications Physics 9 1, 194 (2026).

[19] Cheng-Lin Hong, Luis Colmenarez, Lexin Ding, Carlos L. Benavides-Riveros, and Christian Schilling, "Refining the weighted subspace-search variational quantum eigensolver: compression of ansätze into a single pure state and optimization of weights", arXiv:2306.11844, (2023).

[20] Jing Zhang and Denis Lacroix, "Excited states from ADAPT-VQE convergence path in many-body problems: Application to nuclear pairing problem and <mml:math><mml:msub><mml:mi>H</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:math> molecule dissociation", Physics Letters B 869, 139841 (2025).

[21] Lucien Dupuy and Emmanuel Fromager, "Exact static linear response of excited states from ensemble density functional theory", arXiv:2506.16363, (2025).

[22] Scott E. Smart and Prineha Narang, "Many-body eigenstates from quantum manifold optimization", Physical Review A 110 5, 052430 (2024).

[23] Filip Cernatic and Emmanuel Fromager, "Extended $N$-centered ensemble density functional theory of double electronic excitations", arXiv:2402.07161, (2024).

[24] Francisco M. Fernández, "On the application of the Rayleigh-Ritz method to a projected Hamiltonian", arXiv:2411.14490, (2024).

[25] Yibin Guo, Takis Angelides, Karl Jansen, and Stefan Kühn, "Concurrent VQE for Simulating Excited States of the Schwinger Model", arXiv:2407.15629, (2024).

[26] Ke Liao, "Energy-filtered excited states and real-time dynamics served in a contour integral", arXiv:2409.07354, (2024).

[27] Filip Cernatic and Emmanuel Fromager, "Extended N−centered ensemble density functional theory of double electronic excitations", Journal of Computational Chemistry 45 22, 1945 (2024).

[28] Lucien Dupuy and Emmanuel Fromager, "Exact Static Linear Response of Excited States from Ensemble Density Functional Theory", Journal of Physical Chemistry A 129 39, 9095 (2025).

The above citations are from Crossref's cited-by service (last updated successfully 2026-08-10 18:14:24) and SAO/NASA ADS (last updated successfully 2026-08-10 18:14:26). The list may be incomplete as not all publishers provide suitable and complete citation data.