  State preparation for quantum algorithms is crucial for achieving high
accuracy in quantum chemistry and competing with classical algorithms. The
localized active space unitary coupled cluster (LAS-UCC) algorithm iteratively
loads a fragment-based multireference wave function onto a quantum computer. In
this study, we compare two state preparation methods, quantum phase estimation
(QPE) and direct initialization (DI), for each fragment. We analyze the impact
of QPE parameters, such as the number of ancilla qubits and Trotter steps, on
the prepared state. We find a trade-off between the methods, where DI requires
fewer resources for smaller fragments, while QPE is more efficient for larger
fragments. Our resource estimates highlight the benefits of system
fragmentation in state preparation for subsequent quantum chemical
calculations. These findings have broad applications for preparing
multireference quantum chemical wave functions on quantum circuits,
particularly via QPE circuits.
