Even-handed active space selection in projection-based wavefunction-in-DFT embedding

Abstract

Projection-based embedding offers a simple framework for embedding wavefunction theories in d. functional theory. Underlying this embedding is a heuristic for translating the chem.-intuitive idea of a set of embedded atoms into a set of embedded orbitals on which the embedded wavefunction calcn. will be performed. For single-point calcns., 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 at. population anal. However, during a large geometry change - such as a chem. reaction - the nature of the localized orbitals, as well as their at. populations can change dramatically. This can lead to qual. different embedded orbitals between geometries, resulting in unphys. cusps and even discontinuities in the potential energy surface. In this talk, we present an even-handed framework for localized orbital partitioning that ensures that the span of the embedded orbitals is invariant throughout a geometry coordinate. We illustrate this problem and soln. with a simple example of an SN_2 reaction. We then apply our method to a nitrogen umbrella flip in a cobalt-based CO_2 redn. catalyst and to the binding of CO to a Cu(111) surface.