Active-space construction in single-fragment coupled-cluster embedding for chemical reactions

We benchmark single-fragment quantum embedding, in which electron correlation---at the level of coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)]---is restricted to orbitals associated with a chosen molecular fragment, and the remaining orbitals are frozen at the Hartree-Fock level. We compare several methods for constructing fragment active spaces, and we test their performance on 60 organic reaction energies and three reaction profiles. A common fragment projection operator is used to relate several of the tested orbital constructions. Regional embedding, based on density matrix embedding theory, is shown to give accurate and robust performance when used for both the occupied and virtual active orbitals. For virtual orbitals, fragment-centered projected atomic orbitals and fragment natural orbitals (defined by eigenvectors of a fragment MP2 density matrix) also perform well, but the latter typically retains larger virtual spaces. Composite MP2 corrections yield the largest improvement: combined with fragment natural orbitals, they give average reaction energy errors below 0.3 kcal/mol relative to full-system CCSD(T), with all individual errors below 1 kcal/mol. Reaction profiles show that accurate endpoint energies can conceal substantial barrier errors, emphasizing the importance of approximating omitted correlation.