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The Coupled Cluster (CC) method is used to compute the electronic correlation energy in atoms and molecules and often leads to highly accurate results. However, due to its single-reference nature, standard CC in its projected form fails to…

We propose an orbital optimized method for unitary coupled cluster theory (OO-UCC) within the variational quantum eigensolver (VQE) framework for quantum computers. OO-UCC variationally determines the coupled cluster amplitudes and also…

Strongly Correlated Electrons · Physics 2020-09-23 Wataru Mizukami , Kosuke Mitarai , Yuya O. Nakagawa , Takahiro Yamamoto , Tennin Yan , Yu-ya Ohnishi

A unitary coupled-cluster (UCC) form for the wavefunction in the variational quantum eigensolver has been suggested as a systematic way to go beyond the mean-field approximation and include electron correlation in solving quantum chemistry…

Quantum Physics · Physics 2018-09-12 Ilya G. Ryabinkin , Tzu-Ching Yen , Scott N. Genin , Artur F. Izmaylov

Unitary Coupled Cluster (UCC) theory is a promising variational method for electronic structure calculations, especially for strongly correlated systems and quantum computers. However, its practical application is limited by the steep…

Chemical Physics · Physics 2026-02-05 Prateek Vaish , Brenda Rubenstein

The unitary coupled cluster (UCC) approximation is one of the more promising wave-function ans\"atze for electronic structure calculations on quantum computers via the variational quantum eigensolver algorithm. However, for large systems…

Quantum Physics · Physics 2021-09-30 Jia Chen , Hai-Ping Cheng , James K. Freericks

Quantum computers have emerged as a promising platform to simulate the strong electron correlation that is crucial to catalysis and photochemistry. However, owing to the choice of a trial wave function employed in the popular hybrid…

Chemical Physics · Physics 2023-07-17 Fabijan Pavošević , Ivano Tavernelli , Angel Rubio

We present a four-component relativistic unitary coupled cluster method for molecules. We have used commutator-based non-perturbative approximation using the ''Bernoulli expansion'' to derive an approximation to the relativistic unitary…

Chemical Physics · Physics 2024-03-06 Kamal Majee , Tamoghna Mukhopadhyay , Malaya K. Nayak , Achintya Kumar Dutta

Coupled cluster (CC) methods are among the most accurate methods in quantum chemistry. However, the standard CC linear response formulation is not gauge invariant resulting in errors when modelling properties like optical rotation and…

Chemical Physics · Physics 2018-03-13 Rolf H. Myhre

We present the quantum-selected configuration interaction-tailored coupled-cluster (QSCI-TCC) method, a hybrid quantum-classical scheme that tailors coupled-cluster (CC) theory with a quantum-selected configuration interaction (QSCI) wave…

Chemical Physics · Physics 2025-06-23 Luca Erhart , Yuichiro Yoshida , Wataru Mizukami

Wave functions based on electron-pair states provide inexpensive and reliable models to describe quantum many-body problems containing strongly-correlated electrons, given that broken-pair states have been appropriately accounted for by,…

Chemical Physics · Physics 2021-03-17 Artur Nowak , Ors Legeza , Katharina Boguslawski

Simulating molecules using the Variational Quantum Eigensolver method is one of the promising applications for NISQ-era quantum computers. Designing an efficient ansatz to represent the electronic wave function is crucial in such…

Chemical Physics · Physics 2022-05-09 Dmitry A. Fedorov , Yuri Alexeev , Stephen K. Gray , Matthew Otten

We present a perturbative triples correction to the relativistic quadratic unitary coupled cluster singles and doubles (qUCCSD) method, denoted as qUCCSD[T]. The method builds upon the Hermitian structure of the unitary ansatz and employs a…

Chemical Physics · Physics 2025-10-08 Kamal Majee , Ján Šimunek , Jozef Noga , Achintya Kumar Dutta

Quantum Reservoir Computing (QRC) uses quantum dynamics to efficiently process temporal data. In this work, we investigate a QRC framework based on two coupled Kerr nonlinear oscillators, a system well-suited for time-series prediction…

Quantum Physics · Physics 2026-04-07 Ali Karimi , Hadi Zadeh-Haghighi , Youssef Kora , Christoph Simon

The unitary coupled cluster (UCC) ansatz is a promising tool for achieving high-precision results using the variational quantum eigensolver (VQE) algorithm in the NISQ era. However, results on quantum hardware are thus far very limited and…

Quantum Physics · Physics 2024-11-27 Mark R. Hirsbrunner , Diana Chamaki , J. Wayne Mullinax , Norm M. Tubman

The unitary coupled cluster (UCC) algorithm is one of the most promising implementations of the variational quantum eigensolver for quantum computers. However, for large systems, the number of UCC factors leads to deep quantum circuits,…

Quantum Physics · Physics 2026-02-17 Jeremy Canfield , Dominika Zgid , J K Freericks

Variational quantum eigensolvers (VQE) are among the most promising approaches for solving electronic structure problems on near-term quantum computers. A critical challenge for VQE in practice is that one needs to strike a balance between…

Hybrid quantum-classical approaches offer potential solutions to quantum chemistry problems, yet they often manifest as constrained optimization problems. Here, we explore the interconnection between constrained optimization and generalized…

Quantum Physics · Physics 2025-03-27 Muqing Zheng , Bo Peng , Ang Li , Xiu Yang , Karol Kowalski

One of the commonly used chemical-inspired approaches in variational quantum computing is the unitary coupled-cluster (UCC) ansatze. Despite being a systematic way of approaching the exact limit, the number of parameters in the standard UCC…

Quantum Physics · Physics 2023-06-06 Shashank G Mehendale , Bo Peng , Niranjan Govind , Yuri Alexeev

Unitary coupled cluster (UCC), originally developed as a variational alternative to the popular traditional coupled cluster method, has seen a resurgence as a functional form for use on quantum computers. However, the number of excitors…

Chemical Physics · Physics 2021-01-07 Maria-Andreea Filip , Alex J. W. Thom

Immense interest in quantum computing has prompted development of electronic structure methods that are suitable for quantum hardware. However, the slow pace at which quantum hardware progresses, forces researchers to implement their ideas…

Quantum Physics · Physics 2025-02-26 Ilya G. Ryabinkin , Seyyed Mehdi Hosseini Jenab , Scott N. Genin
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