Reduced-scaling coupled cluster response theory: Challenges and opportunities

We review the current state of reduced-scaling electron correlation methods, particularly coupled-cluster theory for the simulation and prediction of molecular response properties. The successes of local-coupled-cluster and related approaches are well known for reaction energies, thermodynamic constants, dipole moments, and so forth—properties that depend primarily on the quality of the ground-state wave function. However, much more challenging are higher-order properties such as polarizabilities, hyperpolarizabilities, optical rotations, magnetizabilities, and others that also require accurate representation of the derivative of the wave function to external electromagnetic fields. We discuss a range of methods for improving the correlation domains of such perturbed wave functions, including the use of “perturbation-aware” natural orbitals that are customized for the property of interest. In addition, we consider the viability and potential of promising, but still-emerging methods such as stochastic and real-time coupled-cluster approaches, for which the localizability of the field-dependent wave function may be more controllable than for conventional response theory.