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.

A low-cost approach to electronic excitation energies based on the driven similarity renormalization group.

We propose an economical state-specific approach to evaluate electronic excitation energies based on the driven similarity renormalization group truncated to second order (DSRG-PT2). Starting from a closed-shell Hartree-Fock wave function, a model space is constructed that includes all single or single and double excitations within a given set of active orbitals. The resulting VCIS-DSRG-PT2 and VCISD-DSRG-PT2 methods are introduced and benchmarked on a set of 28 organic molecules [M. Schreiber et al., J. Chem. Phys. 128, 134110 (2008)]. Taking CC3 results as reference values, mean absolute deviations of 0.32 and 0.22 eV are observed for VCIS-DSRG-PT2 and VCISD-DSRG-PT2 excitation energies, respectively. Overall, VCIS-DSRG-PT2 yields results with accuracy comparable to those from time-dependent density functional theory using the B3LYP functional, while VCISD-DSRG-PT2 gives excitation energies comparable to those from equation-of-motion coupled cluster with singles and doubles.

An integral-factorized implementation of the driven similarity renormalization group second-order multireference perturbation theory.

We report an efficient implementation of a second-order multireference perturbation theory based on the driven similarity renormalization group (DSRG-MRPT2) [C. Li and F. A. Evangelista, J. Chem. Theory Comput. 11, 2097 (2015)]. Our implementation employs factorized two-electron integrals to avoid storage of large four-index intermediates. It also exploits the block structure of the reference density matrices to reduce the computational cost to that of second-order Møller-Plesset perturbation theory. Our new DSRG-MRPT2 implementation is benchmarked on ten naphthyne isomers using basis sets up to quintuple-ζ quality. We find that the singlet-triplet splittings (ΔST) of the naphthyne isomers strongly depend on the equilibrium structures. For a consistent set of geometries, the ΔST values predicted by the DSRG-MRPT2 are in good agreements with those computed by the reduced multireference coupled cluster theory with singles, doubles, and perturbative triples.

Frozen-Density Embedding Potentials and Chiroptical Properties.

The efficacy of the frozen density embedding (FDE) approach to the simulation of solvent effects is examined for two key chiroptical properties-specific rotation and circular dichroism spectra. In particular, we have investigated the performance of a wave function-theory-in-density-functional-theory (WFT-in-DFT) FDE approach for computing such properties for the small, rigid chiral compound (P)-dimethylallene interacting with up to three water molecules. Although the solvent potential is obtained through DFT, the optical response is computed using coupled cluster linear response theory for mixed electric and magnetic field perturbations. We find that the FDE potential generally yields too small a shift from the isolated molecule as compared to that introduced by the explicit solvent. In one case, the FDE potential fails to reproduce a change in sign of the ORD in which the solute interacts with two solvent molecules. The source of these errors is due primarily to the lack of solvent response to the external field and is analyzed in terms of solvent-solute charge transfer excitations.