Robust, accurate, and efficient: quantum embedding using the Huzinaga level-shift projection operator for complex systems

TL;DR

This paper introduces a robust and accurate quantum embedding method using the Huzinaga level-shift projection operator within an absolutely localized basis, enabling high-precision localized wave function calculations on complex systems with reduced computational cost.

Contribution

The work develops and validates a new quantum embedding approach that accurately handles complex covalent and transition metal bonds, demonstrating systematic improvability and applicability to challenging electronic structures.

Findings

Accurately describes complex covalent and transition metal bonds.

Energy errors less than 1 kcal/mol compared to full-system calculations.

Achieves near full accuracy in adsorption energy calculations with reduced cost.

Abstract

Wave function (WF) in density functional theory (DFT) embedding methods provide 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. In order 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 H$_2$ 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.