Orbital Alignment for Accurate Projection-Based Embedding Calculations along Reaction Paths.

In projection-based embedding (PbE) the subsystem partitioning of a chemical system is based on localized orbitals. We demonstrate how the localization step can lead to inconsistent orbital spaces along reaction paths, with severe consequences for reaction barriers and energies. We propose an orbital alignment procedure that resolves this problem without manual input. The usefulness of this alignment is demonstrated for a reaction benchmark set in combination with a direct orbital selection approach to automatize PbE calculations of double hybrid-in-nonhybrid density functional theory (DFT) and wave-function-in-DFT type for reaction energies and barriers. We show how the embedded calculations are accelerated in comparison to the corresponding supersystem calculations for realistic example reactions, using a new implementation of domain-based local pair natural orbital coupled cluster with single, double, and perturbative triple excitations [DLPNO-CCSD(T ₀ )]. The embedded calculations yield results within an error margin below 4 kJ mol ^-1 for the reaction barrier and energy when compared to the supersystem calculation. The calculations can be executed in a user-friendly, black-box-like fashion with minimal manual input.