Quantum Computing and Coupled-Cluster Theory Enhance Chemical System Calculations.

The research presents a hybrid quantum classical method, QSCI-TCC, which accurately models molecular dissociation, including cases where conventional methods fail. Simulations demonstrate chemically precise results using significantly fewer computational resources than previous implementations, offering a pathway to calculating complex molecular systems.

The accurate modelling of molecular interactions remains a significant challenge in computational chemistry, particularly when dealing with systems exhibiting strong correlation – where electrons interact in complex ways that traditional computational methods struggle to represent. Researchers are increasingly exploring hybrid classical-quantum approaches to overcome these limitations, leveraging the potential of quantum computers to address the exponential scaling of computational cost with system size. A team led by Luca Erhart, Yuichiro Yoshida, and Wataru Mizukami, all from the Center for Quantum Information and Quantum Biology at The University of Osaka, present a novel method in their article, ‘Coupled cluster method tailored by quantum selected configuration interaction’.

Their work details a hybrid scheme, QSCI-TCC, which combines the strengths of selected configuration interaction – a quantum technique for reconstructing electronic states – with the established accuracy of coupled cluster theory, offering a pathway towards simulating complex chemical systems beyond the reach of purely classical computation.

The selected configuration interaction-tailored coupled-cluster (QSCI-TCC) method represents a novel hybrid classical-quantum computational approach designed to address limitations inherent in conventional quantum chemistry. It combines the strengths of selected configuration interaction (SCI), a method for approximating solutions to the Schrödinger equation by focusing on the most important electronic configurations, with tailored coupled-cluster (TCC) theory, a highly accurate but computationally demanding technique. This integration aims to efficiently capture ‘strong correlation’, a phenomenon occurring in molecules where electrons interact strongly and traditional methods struggle to provide accurate descriptions.

Rigorous simulations utilising QSCI-TCC, and its perturbative variant QSCI-TCC(T), demonstrate its performance in challenging chemical scenarios. Specifically, the simultaneous dissociation of the oxygen-hydrogen bond in water (H₂O) and the breaking of the triple bond in nitrogen (N₂) serve as benchmark calculations. These tests reveal that QSCI-TCC and QSCI-TCC(T) yield accurate results even when conventional methods like coupled cluster singles, doubles and perturbative triples [CCSD(T)], a gold standard in quantum chemistry, begin to falter. The improved accuracy arises from the method’s ability to effectively capture ‘static correlation’, arising from near-degeneracy in electronic states, via the QSCI component, subsequently refined by the ‘dynamic correlation’ captured within the coupled-cluster framework.

The implementation of QSCI-TCC leverages both quantum and classical algorithms to minimise computational cost and maximise accuracy. The QSCI component efficiently reconstructs the many-electron state on a classical computer, circumventing the limitations of traditional approaches when dealing with strong correlation. This reconstructed state then informs and constrains the subsequent coupled-cluster calculation, enabling a balanced treatment of both static and dynamic correlation.

Further optimisation of the QSCI component focuses on identifying and utilising only the most important configurations for describing strong correlation, thereby reducing the number of quantum measurements required. This strategic approach minimises the impact of quantum noise and maximises calculation accuracy, even with limited quantum resources. The resulting active-space configuration interaction coefficients, free from additive ‘shot noise’ – a common error in quantum computation, map to fixed cluster amplitudes within the tailored coupled-cluster framework, again enabling a balanced treatment of both static and dynamic correlation.

It is anticipated that QSCI-TCC will become an increasingly important tool for computational chemists and materials scientists. By overcoming the limitations of conventional quantum chemical techniques, it opens up new possibilities for understanding and predicting the behaviour of molecules and materials, potentially leading to breakthroughs in fields such as drug discovery, materials science, and energy research. Ongoing work focuses on extending the QSCI-TCC method to even more complex systems and developing new algorithms to further improve its efficiency and accuracy.