Unitary Coupled Cluster State Preparation using a Sparse Wavefunction Circuit Solver

The variational quantum eigenvalue solver (VQE) using the unitary coupled cluster (UCC) wave function ansatz is a powerful hybrid quantum-classical approach that can employ near-term quantum hardware for computing ground state electronic energies for molecular systems. Unfortunately, even for small molecules, the number of variational parameters and qubits required to minimize the electronic energy is beyond the reach of current quantum computers except for small basis sets. For example, a circuit for C₂ using the UCCSD ansatz and the cc-pVDZ basis set with frozen-core will require over 10,000 variational parameters and a Hilbert space of over determinants. Here, we propose a new paradigm for state preparation where we explore how much of the optimization can be approximately prepared with classical computers to reduce the number of optimization steps performed using a quantum device. By adapting a recent algorithm for the factorized form of the UCC ansatz [J. Chem. Theory Comput. 2021, 17, 841-847] that allows for efficient UCC optimizations on classical hardware, we can study molecular electronic structure problems employing the UCC ansatz with up to 64 qubits. In this presentation, we will present results using our approach and discuss strategies for incorporating this implementation for algorithms involving near-term quantum computers.