  In common with many high-accuracy electronic structure methods, the initiator
adaptation of full configuration interaction quantum Monte Carlo (i-FCIQMC) has
difficulty treating realistic systems with large numbers of electrons. This
barrier has prevented the application of i-FCIQMC to questions of catalysis
that, even for the simplest of models, require high-accuracy modeling of
several features of the electronic structure, such as strong and dynamic
correlation, and localized vs. delocalized bonding. We here present a
fully-quantum embedded version of i-FCIQMC , which we apply to calculate the
bond dissociation energy of an ionic bond (LiH) and a covalent bond (HF)
physisorbed to a benzene molecule. The embedding is performed using a
recently-developed Huzinaga projection operator approach, which affords good
synergy with i-FCIQMC by minimizing the number of orbitals in the calculation.
We find that, without embedding, i-FCIQMC struggles to converge these
calculations due to their substantial system sizes and a lack of error
cancellation between reactants and products. With embedding, the i-FCIQMC
calculation converges straightforwardly to CCSD(T) benchmarks. Our results
suggest that embedded i-FCIQMC will be able treat system sizes well beyond our
current reach (even though embedding introduces an error). We discuss how
embedding might be improved (and thus the introduced error reduced) using
i-FCIQMC energies as benchmarks.
