Density-Based Errors in Mixed-Basis Mean-Field Electronic Structure, with Implications for Embedding and QM/MM Methods

We consider mean-field electronic structure calculations with subsystems that employ different atomic-orbital basis sets. A major source of error arises in charge-manifestation reactions (including ionization, electron attachment, or deprotonation) due to electronic density artifacts at the subsystem interface. The underlying errors in the electronic density can be largely eliminated with Fock-matrix corrections or by avoiding the use of a minimal basis set in the low-level region. These corrections succeed by balancing the electronegativity of atoms at the subsystem interface, much as link-atoms in QM/MM calculations rely upon balancing the electronegativity of atoms in the truncated QM region.