Unitary Block-Correlated Coupled Cluster Ansatz Based on the Generalized Valence Bond Wave Function for Quantum Simulation
Jiaqi Hu, Qingchun Wang, Shuhua Li · Journal of Chemical Theory and Computation · 2025
Strongly correlated (SC) systems present significant challenges for classical quantum chemistry methods. Quantum computing, particularly the variational quantum eigensolver (VQE), offers a promising framework to address these challenges by inherently supporting exponentially large configuration spaces. However, its application to SC systems remains limited due to the single-reference nature of the widely used ansatzes such as unitary coupled cluster (UCC). To address this challenge, we propose the generalized valence bond-based unitary block correlated coupled cluster (GVB-UBCCC) method. This novel ansatz incorporates the multiconfigurational nature of generalized valence bond (GVB) and the accuracy of block correlated coupled cluster (BCCC) methods, making it well-suited for SC systems. We have implemented the GVB-UBCCC method with up to two-block correlation (GVB-UBCCC2) and applied it to investigate ground-state energies for several SC systems, including H 4, the water dimer, N 2 H 2, and S 6, at most described by 24 qubits. Our approach demonstrates that for these systems, GVB-UBCCC2 can achieve more accurate ground-state energies than UCCSD in most cases while requiring only O ( N 2 ) quantum gates and parameters, as opposed to the O ( N 4 ) scaling of UCCSD. The results highlight the effectiveness and potential advantages of GVB-UBCCC in SC systems.