Fully Quantum Embedding with Density Functional Theory for Full Configuration Interaction Quantum Monte Carlo.

We here develop a fully quantum embedded version of initiator full configuration interaction quantum Monte Carlo (-FCIQMC) and apply it to study an ionic bond (lithium hydride, LiH) and a covalent bond (hydrogen flouride, HF) physisorbed to a benzene molecule. The embedding is performed using a recently developed Huzinaga projection operator approach, which affords good synergy with -FCIQMC by minimizing the number of orbitals in the calculation. When considering the dissociation energy of these bonds into closed-shell ionic fragments, we find that -FCIQMC embedded in density functional theory (-FCIQMC-in-DFT) delivers comparable accuracy with coupled cluster singles and doubles with perturbative triples embedded in density functional theory (CCSD(T)-in-DFT). In treating the bond dissociation energy curve of HF, -FCIQMC-in-DFT has improved accuracy over CCSD(T)-in-DFT due to the presence of strong correlation. We discuss the implications of the new -FCIQMC-in-DFT method as applied to bond breaking in catalysis.