Global Optimization of Adsorbate–Surface Structures While Preserving Molecular Identity

As the complexity of atomistic simulations in catalysis and surface science increases, the challenge of manually finding the lowest-energy adsorbate–surface geometries grows significantly. In the current work, a global optimization approach that preserves adsorbate identity is applied to enable the automated search for optimized binding geometries. This technique is based on the minima hopping method developed by Goedecker, but is modified to preserve the molecular identity of adsorbates by the application of a new class of Hookean constraints. These constraints are completely inactive when the adsorbate identity is preserved, but act to restore the adsorbate structure via a Hookean force when the bond length exceeds a threshold distance. Additionally, a related Hookean constraint has been developed to prevent adsorbates (particularly such adsorbates as CO and CH₂O that have stable gas-phase forms) from volatilizing during the molecular dynamics portion of the minima hopping technique. This combination, referred to herein as the constrained minima hopping method, was tested for its suitability in finding the minimum-energy binding configuration for a set of 17 C _x H _y O _z adsorbates on a stepped Cu fcc(211) surface and in all cases found the global minima in comparable or fewer steps than the previous brute force methodologies. It is expected that methods such as this will be crucial to finding low-energy states in more complex systems, such as those with high coverages of adsorbed species or in the presence of explicit solvent molecules.