Broadening the Scope of the ETS-NOCV Scheme: A Versatile Implementation in ORCA

We report a comprehensive implementation of the Extended Transition State-Natural Orbitals for Chemical Valence (ETS-NOCV) scheme in the orca quantum chemistry package. This implementation broadens the applicability of ETS-NOCV by supporting a wide range of electronic structure methods, including hybrid and double-hybrid density functionals, as well as modern semiempirical and composite approaches such as HF-3c and r²SCAN-3c. The implementation is fully parallelized, allowing for the routine analysis of bonding interactions in large molecular systems. We benchmark the method across a set of chemically diverse systems, ranging from noncovalent dimers to dirhodium-carbene complexes, with a particular focus on the performance of semiempirical and double-hybrid functionals, which had not been previously available within this framework. The results highlight the physical interpretability of the method and the computational efficiency of our implementation, providing practical guidelines for selecting appropriate levels of theory in different bonding scenarios.