Towards Accurate Description of Chemical Reaction Energetics by Using Variational Quantum Eigensolver: A Case Study of the C2v Quasi-Reaction Pathway of Beryllium Insertion to H2 Molecule

Abstract

Variational quantum eigensolver (VQE)-based quantum chemical calculations of atoms and molecules have been extensively studied as a computational model using noisy intermediate-scale quantum devices. VQE uses a parametrized quantum circuit defined through an "ansatz" to generate the approximated wave functions, and thus appropriate choice of an ansatz is the most important step. Because most of chemistry problems focus on the energy difference between two electronic states or structures, calculating the total energies in different molecular structures with the same accuracy is essential to correctly understand chemistry and chemical processes. Here we applied numerical simulations of VQE to the quasi-reaction pathway of Be insertion to H2 molecule, which is the representative systems of both dynamical and static electron correlation effects are prominent. Our numerical simulations revealed that the energy calculated using the conventional unitary coupled cluster (UCC) ansatz exhibits large discrepancy from the full-configuration interaction (CI) value around the point where an avoided crossing occurs. We demonstrated that the multireference unitary coupled cluster with partially generalized singles and doubles (MR-UCCpGSD) ansatz can give more reliable results in respect of total energy and the overlap with the full-CI solution, insisting importance of multiconfigurational treatments in the calculations of strongly correlated systems.