Level-Shifted Embedded Cluster Method for Modeling the Chemistry of Metal Oxides

The
embedded cluster method has been used extensively in the study
of the chemical and physical properties of metal oxides. This method
has been a popular tool due to its relatively high accuracy and low
computational cost. An even more promising option may entail integrating
the embedded cluster method with the combined quantum mechanical and
molecular mechanical (QM/MM) approach, thereby enabling further consideration
of interactions within the entire system for superior results. We
aim to accurately model the chemistry of metal oxides using this combined
scheme. Here, using the prototypical MgO(100) surface as a test system,
with Mg9O14 as the cluster in the quantum mechanical
region, we show that the embedded cluster with untailored boundary
effective core potentials (ECPs) can have frontier orbital energy
levels that substantially deviate from the quantum mechanical reference
results. This occurs even when Mg9O9, which
retains the stoichiometry of MgO, is used as the cluster in the quantum
mechanical region. As a result, the chemical properties of the embedded
cluster models differ from those of the quantum mechanical reference
model. To address this issue, we propose a new variant of the embedded
cluster method called the level-shifted embedded cluster (LSEC) method,
which allows the energy levels to be shifted to match the reference
levels by tuning the boundary ECPs. Our validation calculations on
the adsorption of various adsorbates with different properties on
the MgO(100) surface show that the overall performance of QM/MM with
the LSEC method is excellent for the adsorption energies, geometries,
and charge properties. The excellent performance holds for both the
nonstoichiometric and stoichiometric clusters (i.e., Mg9O14 and Mg9O9, respectively), demonstrating
the robustness of the LSEC method. We expect that the LSEC method
can be combined with QM/MM or used separately for future chemical
studies of metal oxides and other ionically bonded systems.