Complexes and Reaction Energetics of Hydrogenation of Small Li Clusters using VQE-based Quantum Chemical Calculations*

We utilize the variational quantum eigensolver (VQE) using a unitary coupled cluster with singles and doubles (UCCSD) ansatz to simulate a series of LiH_n molecules (n=1-3), including their singly charged ions. We consider the performance using a simulator by systematically increasing system size, providing data to compare against future VQE calculations with advances in methods. We show how VQE-UCCSD is comparable to classical CCSD for small systems like LiH with respect to full configuration interaction (FCI). However for larger systems such as linear neutral LiH₂, the UCCSD – FCI ground state energy difference is much smaller than CCSD – FCI. Algorithmic advances have yielded protocols that allow the treatment of chemical problems that are intractable using brute-force VQE. In particular, we numerically test the HF-Embedding method for hydrogenating Li₂, Li₃, and Li₄ clusters using STO-6G basis sets. The predicted energy profiles are in semi-quantitative agreement with highly accurate CCSD/FCI methods.