Accurate spin-densities based on the domain-based local pair-natural orbital coupled-cluster theory

Exploiting locality in the electron correlation reduces the computational cost for solving the Coupled-Cluster (CC) equations. This is important for making CC theory applicable to routine computational chemistry applications where it promises to deliver results of "gold-standard" quality. Recently, we have proposed a series of CC formulations in the domain-based local pair-natural orbital framework [DLPNO-coupled-cluster with singles and doubles (CCSD) and DLPNO-coupled-cluster singles and doubles with perturbative triples] which are designed to reproduce approximately 99.9% of the canonical correlation energy. In our previous work, the DLPNO-CCSD method has been extended to the high-spin open-shell reference and shown to possess comparable accuracy to the closed-shell counterpart [M. Saitow et al., J. Chem. Phys. 146, 164105 (2017)]. The so-called Λ-equations have been formulated in the DLPNO framework for the closed-shell species as an exact derivative of the DLPNO-CCSD Lagrangian with respect to the PNO-based cluster amplitudes [D. Datta et al., J. Chem. Phys. 145, 114101 (2016)]. In this paper, we extend the DLPNO-based Lagrangian scheme to the high-spin open-shell reference cases, thus enabling the accurate computation of the electron- and spin-densities for large open-shell species. We apply this newly developed approach to various first-order electronic and magnetic properties such as isotropic and anisotropic components in the hyperfine coupling interactions and the electric field gradient. We demonstrate that the DLPNO-CCSD results converge toward the respective canonical CC density and also that the DLPNO-CCSD-based properties are more accurate than the conventional density functional theory (DFT) results in real-life applications. The additional computational cost is not more than one energy evaluation in the DLPNO-CCSD framework.