Coupled-cluster treatment of complex open-shell systems: the case of single-molecule magnets.

We investigate the reliability of two cost-effective coupled-cluster methods for computing spin-state energetics and spin-related properties of a set of open-shell transition-metal complexes. Specifically, we employ the second-order approximate coupled-cluster singles and doubles (CC2) method and projection-based embedding that combines equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) with density functional theory (DFT). The performance of CC2 and EOM-CCSD-in-DFT is assessed against EOM-CCSD.

On the performance of second-order approximate coupled-cluster singles and doubles methods for non-valence anions.

We investigate the capability of several variants of the second-order approximate coupled-cluster singles and doubles (CC2) method to describe dipole-bound, quadrupole-bound, and correlation-bound molecular anions. The binding energy of anions formed by electron attachment to closed-shell molecules is computed using the electron attachment variant of CC2 (EA-CC2), whereas anions with a closed-shell ground state are treated with the standard CC2 method that preserves the number of particles. We find that EA-CC2 captures the binding energies of dipole-bound radical anions quite well, whereas results for other types of non-valence anions are less reliable.

A spin-flip variant of the second-order approximate coupled-cluster singles and doubles method.

We report an implementation of a spin-flip variant of the second-order approximate coupled-cluster singles and doubles (CC2) method. The resolution-of-the-identity approximation or, alternatively, Cholesky decomposition are applied to the electron repulsion integrals. We illustrate the performance of the new method by constructing potential energy curves of H and HF and by computing singlet-triplet splittings for various diradicals including some binuclear copper complexes that are of interest as molecular magnets.