Second-Order Active-Space Embedding Theory

Quantum embedding schemes are a promising
way to extend multireference
computations to large molecules with strong correlation effects
localized on a small number of atoms. This work introduces a second-order
active-space embedding theory [ASET(2)] which improves upon mean-field
frozen embedding by treating fragment–environment interactions
via an approximate canonical transformation. The canonical transformation
employed in ASET(2) is formulated using the driven similarity renormalization
group. The ASET(2) scheme is benchmarked on the NN bond dissociation
in pentyldiazene, the S0 to S1 excitation in
1-octene, and the interaction energy of the O2–benzene
complex. The ASET(2) explicit treatment of fragment–environment
interactions beyond the mean-field level generally improves the accuracy
of embedded computations, and it becomes necessary to achieve an accurate
description of excitation energies of 1-octene and the singlet–triplet
gap of the O2–benzene complex.