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We introduce the atomic valence active space (AVAS), a simple andwell-defined automated technique for constructing active orbital spaces for usein multi-configuration and multireference (MR) electronic structurecalculations. Concretely, the technique constructs active molecular orbitalscapable of describing all relevant electronic configurations emerging from atargeted set of atomic valence orbitals (e.g., the metal d orbitals in acoordination complex). This is achieved via a linear transformation of theoccupied and unoccupied orbital spaces from an easily obtainablesingle-reference wavefunction (such as from a Hartree-Fock or Kohn-Shamcalculations) based on projectors to targeted atomic valence orbitals. Wediscuss the premises, theory, and implementation of the idea, and several ofits variations are tested. To investigate the performance and accuracy, wecalculate the excitation energies for various transition metal complexes intypical application scenarios. Additionally, we follow the homolytic bondbreaking process of a Fenton reaction along its reaction coordinate. While thedescribed AVAS technique is not an universal solution to the active spaceproblem, its premises are fulfilled in many application scenarios of transitionmetal chemistry and bond dissociation processes. In these cases the techniquemakes MR calculations easier to execute, easier to reproduce by any user, andsimplifies the determination of the appropriate size of the active spacerequired for accurate results.

Automated construction of molecular active spaces from atomic valence orbitals