Even-handed subsystem selection in projection-based wavefunction-in-DFT embedding

Projection-based embedding offers a simple framework for embedding wavefunction theories in density functional theory. Underlying this embedding is a heuristic for translating the chemically-intuitive idea of a set of embedded atoms into a set of embedded orbitals on which the wavefunction calculation will be performed. For single-point energy calculations, a successful heuristic has been to first localize the occupied Kohn-Sham orbitals, and then assign these localized orbitals to the embedded region based on atomic population analysis. However, for chemical reactions involving large geometry changes, the nature of the localized orbitals—as well as their atomic populations—can change dramatically. This can lead to qualitatively different embedded orbitals between geometries, resulting in unphysical cusps and even discontinuities in the potential energy surface. In this talk, we present an even-handed framework for localized orbital partitioning that ensures invariance of the span of the embedded orbitals throughout a geometry coordinate. We illustrate this problem and its solution with a simple example of an SN2 reaction. We then apply our method to a nitrogen umbrella flip in a cobalt-based CO2 reduction catalyst and to the binding of CO to a Cu(111) surface.