A Combined Selected Configuration Interaction and Many-Body Treatment of Static and Dynamical Correlation in Oligoacenes

We
have combined our adaptive configuration interaction (ACI) [J.
B. Schriber and F. A. Evangelista, J. Chem. Phys. 2016, 144, 161106] with a density-fitted
implementation of the second-order perturbative multireference-driven
similarity renormalization group (DSRG-MRPT2) [K. P. Hannon, C. Li,
and F. A. Evangelista, J. Chem. Phys. 2016, 144, 204111]. We use ACI reference wave functions
to recover static correlation for active spaces larger than the conventional
limit of 18 orbitals. The dynamical correlation is computed using
the DSRG-MRPT2 method to yield a complete treatment of electron correlation.
We apply the resulting method, ACI-DSRG-MRPT2, to predict singlet–triplet
gaps, metrics of open-shell character, and spin–spin correlation
functions for the oligoacene series (2–7 rings). Our computations
employ active spaces with as many as 30 electrons in 30 orbitals and
up to 1350 basis functions, yielding gaps that are in good agreement
with available experimental results. Large bases and reference relaxation
lead to a significant reduction in the estimated radical character
of the oligoacenes, with respect to previous valence-only treatments
of correlation effects.