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arXiv:2509.13826

Quantum Simulations of Battery Electrolytes with VQE-qEOM and SQD: Active-Space Design, Dissociation, and Excited States of LiPF$_6$, NaPF$_6$, and FSI Salts

Sk Mujaffar Hossain, Seung‐Cheol Lee, Satadeep Bhattacharjee

Published Sep 17, 2025• 0 citations
arXivHTMLPDF

Abstract

Accurate prediction of excited states in battery electrolytes is central to understanding photostability, oxidative stability, and degradation. We employ hybrid quantum-classical algorithms -- the Variational Quantum Eigensolver (VQE) for ground states combined with the quantum equation of motion (qEOM) for vertical singlet excitations -- to study LiPF$_6$, NaPF$_6$, LiFSI, and NaFSI. Compact active spaces were constructed from frontier orbitals, mapped to qubits, and reduced via symmetry tapering and commuting-group measurements to lower sampling cost. Within $\sim$10-qubit models, VQE-qEOM agrees closely with exact diagonalization of the same Hamiltonians, while sample-based quantum diagonalization (SQD) in larger active spaces recovers near-exact (subspace-FCI) energies. The spectra display clear anion and cation trends: PF$_6$ salts exhibit higher first-excitation energies (e.g., LiPF$_6$ $\approx$13.2 eV) and a compact three-state cluster at 12-13 eV, whereas FSI salts show substantially lower onsets ($\approx$8-9 eV) with a near-degenerate (S$_1$,S$_2$) followed by S$_3$ $\sim$1.3 eV higher. Substituting Li$^+$ with Na$^+$ narrows the gap by $\sim$0.4-0.8 eV within each anion family. Converting S$_1$ to wavelengths places the onsets in the deep-UV (LiPF$_6$ $\sim$94 nm; NaPF$_6$ $\sim$100 nm; LiFSI $\sim$141 nm; NaFSI $\sim$148 nm). All results pertain to isolated species or embedded clusters appropriate to the NISQ regime; solvent shifts can be incorporated a posteriori via classical $Δ$-solvation or static embedding. These results demonstrate that current quantum algorithms can deliver chemically meaningful excitation and binding trends for realistic electrolyte motifs and provide quantitative baselines to guide electrolyte screening and design.

DOI
10.48550/arxiv.2509.13826

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ABSTRACT Accurate prediction of excited states in battery electrolytes is crucial for understanding photostability, oxidative stability, and degradation. We employ hybrid quantum‐classical algorithms–the Variational Quantum Eigensolver (VQE) for ground states and the quantum equation of motion (qEOM) for vertical singlet excitations to study , , LiFSI, and NaFSI. Compact active spaces from frontier orbitals were mapped to qubits and reduced via symmetry tapering and commuting‐group measurements to lower sampling cost. Within 10‐qubit models, VQE‐qEOM agrees closely with exact diagonalization, while sample‐based quantum diagonalization (SQD) in larger spaces recovers near‐exact (subspace‐FCI) energies. Spectra show clear anion‐cation trends within the VQE‐qEOM framework: salts have higher first‐excitation energies ( 13.2 eV) and a compact three‐state cluster at 12–13 eV, whereas FSI salts exhibit lower onsets (8–9 eV) with nearly degenerate and states and a higher separated by 1.3 eV. Independent TDDFT calculations yield systematically lower absolute excitation energies but reproduce the same anion‐ and cation‐dependent trends, confirming that the relative ordering and physical interpretation of the quantum results are robust. Replacing with narrows the gap by 0.4–0.8 eV per anion family. Converting to wavelengths places onsets in the deep UV ( 94 nm; 100 nm; LiFSI 141 nm; NaFSI 148 nm). Results for isolated species or embedded clusters are NISQ‐feasible, with solvent shifts incorporable via classical ‐solvation. Current quantum algorithms capture excitation trends, advancing electrolyte design.
Accurate prediction of reaction barriers and transition states is central to understanding electrolyte degradation pathways in battery systems, yet existing approaches face significant computational trade-offs. Classical quantum-chemistry methods like density functional theory (DFT) scale favorably but often miss crucial multi-reference character at transition states, while high-accuracy methods such as CCSD(T) become prohibitively expensive for realistic systems. Recent quantum algorithmic approaches, including Variational Quantum Eigensolver (VQE) and Quantum Subspace Diagonalization (QSD), offer pathways toward near-chemical accuracy but suffer from manual active-space selection, excessive measurement overhead, and lack of systematic refinement protocols. We introduce Adaptive Quantum Subspace Diagonalization (A-QSD), a comprehensive pipeline that addresses these limitations through automated chemically-aware active-space selection, compact matrix-element estimation via Hadamard test and classical shadows, and iterative quantum imaginary-time evolution (QITE) preconditioning. Targeting ethylene glycol oxidation pathways relevant to battery electrolyte stability, A-QSD achieves transition-state energy accuracy within 0.1 eV of high-level references while maintaining resource requirements of <=18 qubits and <=700 seconds runtime per reaction point. This work demonstrates the first systematic quantum approach for mapping realistic electrolyte reaction coordinates, establishing a practical framework for quantum-enhanced materials discovery in energy storage applications.
Accurately capturing electron correlation in large-scale molecular systems remains one of the foremost challenges in quantum chemistry and a primary driver for the development of quantum algorithms. Classical configuration-interaction methods, while rigorous, suffer from exponential scaling, rendering them impractical for large or strongly correlated systems. Overcoming this limitation is central to realizing the promise of quantum computing in chemistry. Here, we investigate the pyridine-Li ion complex using three quantum algorithms: the variational quantum eigensolver (VQE), the subspace quantum diagonalization (SQD) method, and the recently introduced handover iterative VQE (HI-VQE). Our results demonstrate how new generations of hybrid quantum-classical frameworks overcome the scalability and noise sensitivity that constrain conventional VQE approaches. SQD and HI-VQE achieve ground-state energy calculations for problem sizes inaccessible to classical computation, marking a clear advance toward quantum advantage. In particular, HI-VQE enables calculations within active spaces as large as (24e,22o), requiring 44 qubits-well beyond the reach of classical CASCI and VQE. This capability provides a systematic pathway for incorporating increasing numbers of electrons into quantum treatment, thereby approaching exact molecular energies. Importantly, both SQD and HI-VQE exhibit robustness against hardware noise, a critical improvement over earlier approaches. By enabling quantum simulations of molecular systems previously deemed intractable, SQD and HI-VQE offer a realistic route toward practical quantum advantage in computational chemistry. The comparison between HI-VQE and SQD shows that optimizing circuit parameters is crucial for accurate simulation.
We present a large-scale experimental study of quantum-computing-based molecular simulation carried out on IQM's Sirius 24-qubit superconducting processor, utilizing up to 16 operational qubits. The work employs Sample-based Quantum Diagonalization (SQD) together with the Local Unitary Cluster Jastrow (LUCJ) ansatz to estimate ground-state energies for a set of benchmark molecules, including H2_22​, LiH, BeH2_22​, H2_22​O, and NH3_33​. In addition, we introduce a Linear-CNOT variant of the Unitary Coupled-Cluster Singles and Doubles (LCNot-UCCSD) ansatz within the SQD workflow, trading higher circuit depth for reduced classical preprocessing. A comparison between these ansätze is provided, clarifying their respective strengths, limitations, and suitability for near-term quantum hardware. We further explore potential energy landscapes through 1D scans for H2_22​ and HeH+^++ using both STO-3G and 6-31G basis sets, and for LiH and BeH2_22​ in STO-3G. Extending beyond this, we demonstrate the experimental construction of a full 2D potential energy surface for the water molecule on quantum hardware, mapped over a 32 ×\times× 32 grid in bond length and bond angle. To move beyond small benchmark systems, we combine SQD(LUCJ) with Density Matrix Embedding Theory (DMET) to compute active-space energies for a set of ligand-like molecules, as well as the pharmacologically relevant amantadine system. Across all studies, the majority of quantum-computed energies agree with reference FCI results, as well as with DMET-CASCI energies for embedded systems, to within chemical accuracy for the chosen basis sets. These results demonstrate the reliability of sample-based diagonalization approaches and underscore the potential of hybrid embedding strategies for extending quantum simulations to increasingly complex molecular systems, while also highlighting their practicality on current IQM quantum hardware.
Advancing battery materials is crucial for sustainable infrastructure, EVs and renewable energy storage. By achieving polynomial scaling for exact electronic structure simulations of materials, quantum computing could emerge as an invaluable tool in this field. In this work, we address two fundamental aspects related to the application of quantum algorithms to lithium diffusion in solid state electrolytes, which have the potential to significantly improve safety and performance of current battery technologies. First of all, we discuss the need for the higher level of accuracy provided by quantum computing. Classical computational methods routinely applied to compute the properties of solid state electrolytes are based on semi-local approximations of density functional theory (DFT). Considering cluster models representative of the properties of the bulk Li10GeP2S12(LGPS) electrolyte, we benchmark several functionals, including state-of-the-art hybrid approximations and compare the results to high-level wavefunction methods. The classical results for lithium diffusion barriers exhibit a variability exceeding the chemical accuracy threshold required for experimentally relevant precision, underscoring the necessity for exact quantum solutions as a reference. Secondly, we perform quantum simulations for lithium diffusion barriers using small active spaces of the cluster models and use those results to extrapolate the minimal resources required for a full simulation of bulk LGPS. Specifically, the variational quantum eigensolver (VQE) with a quantum number preserving ansatz and the quantum Krylov (qK) method are considered. The resource estimations for VQE show that a few hundred qubits and less than one million two-qubit gates could be sufficient to simulate LGPS. For qK the number of required two-qubit gates increases by orders of magnitude. However, for large models it should also be considered that qK avoids the complex non-linear optimization problem involved in VQE. Future advancements in quantum algorithms will be essential for bringing simulations of materials closer to realization.
The variational quantum eigensolver (VQE) is a hybrid quantum-classical algorithm designed for noisy intermediate-scale quantum (NISQ) hardware to estimate eigenvalues of many-body Hamiltonians. Unlike fully quantum approaches such as quantum phase estimation (QPE), VQE trades deep coherent circuits for repeated state preparation, measurement, and classical optimization, making it more compatible with limited qubit counts and finite coherence times. Recent developments have focused on reducing quantum resource requirements while retaining chemically meaningful wavefunction structure. In this paper, we examine recent progress in VQE methods for molecular electronic structure with an emphasis on three themes: (i) strategies for circuit and ansatz complexity reduction, including adaptive and selectively screened approaches, (ii) chemically motivated workflows that combine VQE with orbital optimization, fragmentation, and localized active-space ideas to better address strong correlation, and (iii) extensions of VQE to excited-state calculations. Throughout, we emphasize the tradeoffs among parameter count, gate depth, symmetry preservation, measurement overhead, and classical preprocessing, and discuss where these approaches may become most useful for chemically challenging active spaces. We also highlight benchmarking considerations for assessing both accuracy and resource requirements, and conclude with a perspective on regimes in which VQE may offer the greatest long-term value, particularly multireference active spaces and low-lying excited-state manifolds.