Robust, Accurate, and Efficient: Quantum Embedding Using the Huzinaga Level-Shift Projection Operator for Complex Systems

Using wave function (WF) in density
functional theory (DFT) embedding
methods provides a framework for performing localized, high-accuracy
WF calculations on a system, while not incurring the full computational
cost of the WF calculation on the full system. To effectively partition
a system into localized WF and DFT subsystems, we utilize the Huzinaga
level-shift projection operator within an absolutely localized basis.
In this work, we study the ability of the absolutely localized Huzinaga
level-shift projection operator method to study complex WF and DFT
partitions, including partitions between multiple covalent bonds,
a double bond, and transition-metal–ligand bonds. We find that
our methodology can accurately describe all of these complex partitions.
Additionally, we study the robustness of this method with respect
to the WF method, specifically where the embedded systems were described
using a multiconfigurational WF method. We found that the method is
systematically improvable with respect to both the number of atoms
in the WF region and the size of the basis set used, with energy errors
less than 1 kcal/mol. Additionally, we calculated the adsorption energy
of H2 to a model of an iron metal–organic framework
(Fe-MOF-74) to within 1 kcal/mol compared to CASPT2 calculations performed
on the full model while incurring only a small fraction of the full
computational cost. This work demonstrates that the absolutely localized
Huzinaga level-shift projection operator method is applicable to very
complex systems with difficult electronic structures.