Surface adsorption at the thermodynamic limit using periodic DLPNO-MP2 theory: A study of CO on MgO at dilute and dense coverages

TL;DR

The study demonstrates that periodic DLPNO-MP2 can efficiently model CO adsorption on MgO(001) at the thermodynamic limit, validating the dilute-limit adsorption energy against established benchmarks and extending the analysis to dense CO coverages. By employing Megacell-DLPNO-MP2 across large supercells, the authors capture surface-slab convergence and quantify how lateral CO-CO interactions modulate adsorption energies as coverage increases, observing a reduction in binding strength toward full monolayer due to repulsions. The work highlights the method's scalability for complex adsorption problems and confirms that dense-coverage behavior can be described within a consistent correlated-wavefunction framework, despite basis-set limitations. Overall, the approach offers a robust path to investigate realistic, defect-aware, and densely populated surface interfaces with quantum-chemical accuracy on thermodynamic-length scales.

Abstract

We apply periodic domain-based local pair natural orbital second-order Møller--Plesset perturbation theory (DLPNO-MP2) to probe the adsorption energy of CO on MgO(001), the consensus toy model system for surface adsorption. A number of robust correlated wavefunction methods now achieve excellent agreement with experiment for the adsorption of a single CO molecule onto the MgO surface. However, studies probing denser coverage ratios are scarce because of the increased computational expense and the larger configuration space to optimize. We leverage the computational efficiency of periodic DLPNO-MP2 to perform simulations beyond a single unit cell. By using large supercells, we highlight the importance of accurately representing the thermodynamic limit of the surface, and demonstrate in turn that different coverage ratios can be consistently probed. In the dilute regime, we show that adsorption energies obtained from periodic DLPNO-MP2 agree with existing benchmarks. We then obtain adsorption energies at increasing densities approaching full monolayer coverage. Our results show a reduction in binding strength at full coverage, agreeing with experimental observations, which is explained by the increasing lateral repulsions between the COs. This study demonstrates the efficacy of periodic DLPNO-MP2 for probing increasingly sophisticated adsorption systems at the thermodynamic limit.

Figures (5)

• Figure 1: Unit cell structures for CO adsorption on an MgO surface corresponding to coverage ratios of $\Theta=\frac{1}{4}$ (top) and $\Theta = \frac{1}{9}$ (bottom). The unit cells are depicted on the left, the $3{\times}3$ supercells on the right where the central cell CO molecule is highlighted.
• Figure 2: The contributions to the adsorption energy of CO on the MgO surface, for any given coverage ratio. The lateral interactions of the gas phase CO molecules must be added to $E_{\mathrm{int}}$ to properly distinguish adsorption energies at different coverage ratios.
• Figure 3: Hartree--Fock (Top) and MP2 correlation energies (Bottom) for $E_{\mathrm{int,coh}}$, at three dilute coverage ratios. Results using the modified 'DZ' and 'TZ' basis sets are shown, and complete basis set estimate is provided for the MP2 correlation energies.
• Figure 4: $2\times2$ and $4\times4$ surface slab unit cells used to probe adsorption coverage ratios towards the dense regime.
• Figure 5: Total CO adsorption energies on the MgO surface at varying coverage ratios up to full monolayer coverage, using the $2\cdot2$ surface slab unit cell, including error bars. Hartree--Fock contributions, in the 'TZ' basis, are plotted as diamonds. MP2 correlation energies contributions, extrapolated to the complete PNO space and basis set limits, are plotted as circles.