Benchmarking the Semi-Stochastic CC(P;Q) Approach for Singlet-Triplet Gaps in Biradicals

Abstract: We recently proposed a semi-stochastic approach to converging high-level coupled-cluster (CC) energetics, such as those obtained in the CC calculations with singles, doubles, and triples (CCSDT), in which the deterministic CC($P$;$Q$) framework is merged with the stochastic configuration interaction Quantum Monte Carlo propagations [J. E. Deustua, J. Shen, and P. Piecuch, Phys. Rev. Lett. 119, 223003 (2017)]. In this work, we investigate the ability of the semi-stochastic CC($P$;$Q$) methodology to recover the CCSDT energies of the lowest singlet and triplet states and the corresponding singlet-triplet gaps of biradical systems using methylene, ${\rm (HFH)}^{-}$, cyclobutadiene, cyclopentadienyl cation, and trimethylenemethane as examples.