Quantum-inspired algorithm for the factorized form of unitary coupled cluster theory

J Chen, HP Cheng, JK Freericks - Journal of Chemical Theory and …, 2021 - ACS Publications
Journal of Chemical Theory and Computation, 2021ACS Publications
The factorized form of unitary coupled cluster theory (UCC) is a promising wave-function
ansatz for the variational quantum eigensolver algorithm. Here, we present a quantum-
inspired classical algorithm for UCC based on an exact operator identity for the individual
UCC factors. We implement this algorithm for calculations of the H10 linear chain and the
H2O molecule with single and double ζ basis sets to provide insights into UCC as a wave-
function ansatz. We find that for weakly correlated molecules, the factorized form of the UCC …
The factorized form of unitary coupled cluster theory (UCC) is a promising wave-function ansatz for the variational quantum eigensolver algorithm. Here, we present a quantum-inspired classical algorithm for UCC based on an exact operator identity for the individual UCC factors. We implement this algorithm for calculations of the H10 linear chain and the H2O molecule with single and double ζ basis sets to provide insights into UCC as a wave-function ansatz. We find that for weakly correlated molecules, the factorized form of the UCC provides similar accuracy to conventional coupled cluster theory (CC); for strongly correlated molecules, where CC often breaks down, UCC significantly outperforms the configuration interaction (CI) ansatz. As a result, the factorized form of the UCC is an accurate, efficient, and reliable electronic structure method in both the weakly and strongly correlated regions. This classical algorithm now allows robust benchmarking of anticipated results from quantum computers and application of coupled-cluster techniques to more strongly correlated molecules.
ACS Publications
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