Local correlation domains for coupled cluster theory: optical rotation and magnetic-field perturbations

An approach is described for selecting local-correlation orbital domains appropriate for computing response properties such as optical rotation using frequency-dependent coupled-cluster linear-response theory. This scheme is an extension of our earlier idea [N. J. Russ and T. D. Crawford, Chem. Phys. Lett., 2004, 400, 104] based on an atom-by-atom decomposition of the coupled-perturbed Hartree-Fock (CPHF) response of the component molecular orbitals to external electric and magnetic fields. We have applied this domain-selection scheme to a series of chiral molecules, including pseudo-linear structures (hydrogen molecule helices, fluoroalkanes, and [n] triangulanes), cage-like structures (beta-pinene, methylnorbornanone, and bisnoradamantan-2-one), and aromatic rings (1-phenylethanol). We find that the crossover points between the canonical- and local-correlation approaches are larger than for the conventional Boughton-Pulay domain scheme, in agreement with our earlier analysis of dipole-polarizabilities. Localization errors are reasonably small (a few percent) for pseudo-linear structures with domain sizes of approximately six to eight atoms. Cage-like molecules are significantly more problematic, requiring natural domain sizes of ten or more to obtain the most reliable localization errors.