  We introduce the atomic valence active space (AVAS), a simple and
well-defined automated technique for constructing active orbital spaces for use
in multi-configuration and multireference (MR) electronic structure
calculations. Concretely, the technique constructs active molecular orbitals
capable of describing all relevant electronic configurations emerging from a
targeted set of atomic valence orbitals (e.g., the metal d orbitals in a
coordination complex). This is achieved via a linear transformation of the
occupied and unoccupied orbital spaces from an easily obtainable
single-reference wavefunction (such as from a Hartree-Fock or Kohn-Sham
calculations) based on projectors to targeted atomic valence orbitals. We
discuss the premises, theory, and implementation of the idea, and several of
its variations are tested. To investigate the performance and accuracy, we
calculate the excitation energies for various transition metal complexes in
typical application scenarios. Additionally, we follow the homolytic bond
breaking process of a Fenton reaction along its reaction coordinate. While the
described AVAS technique is not an universal solution to the active space
problem, its premises are fulfilled in many application scenarios of transition
metal chemistry and bond dissociation processes. In these cases the technique
makes MR calculations easier to execute, easier to reproduce by any user, and
simplifies the determination of the appropriate size of the active space
required for accurate results.
