Hybrid Quantum Algorithms for Computational Chemistry: Application to the Pyridine-Li ion Complex

TL;DR

This paper explores hybrid quantum algorithms like VQE, SQD, and HI-VQE for large-scale molecular systems, demonstrating their potential to achieve quantum advantage in computational chemistry by handling larger active spaces with noise robustness.

Contribution

It introduces and compares advanced hybrid quantum algorithms, showing their ability to simulate larger molecules with improved scalability and noise resilience, surpassing classical methods.

Findings

HI-VQE handles active spaces up to (24e,22o) with 44 qubits.

SQD and HI-VQE outperform classical methods in large problem sizes.

Both algorithms show robustness against hardware noise.

Abstract

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.