  Compared to common density functionals, ab initio wave function methods can
provide greater reliability and accuracy, which could prove useful when
modeling adsorbates or defects of otherwise periodic systems. However, the
breaking of translational symmetry necessitates large supercells that are often
prohibitive for correlated wave function methods. As an alternative, we
introduce the regional embedding approach, which enables correlated wave
function treatments of only a target fragment of interest through small,
fragment-localized orbital spaces constructed using a simple overlap criterion.
Applications to the adsorption of water on lithium hydride, hexagonal boron
nitride, and graphene substrates show that regional embedding combined with
focal point corrections can provide converged CCSD(T) (coupled cluster)
adsorption energies with very small fragment sizes.
