Fig. 1: Examples of possible active and inactive space selections for an isolated and periodic system alike.
From: A general framework for active space embedding methods with applications in quantum computing
The left column depicts the qualitative molecular orbitals of a water molecule. The right column depicts the molecular orbitals localized around the positively charged boron vacancy in hexagonal boron nitride. In both cases, only a small number of orbitals is included in the active space: for water, it is spanned by the HOMO-LUMO pair, while for boron nitride, by the localized defect orbitals.
