The orbital picture of the first dipole hyperpolarizability from many-body response theory
We present an approach for obtaining a molecular orbital picture of the first dipole hyperpolarizability (beta) from correlated many-body electronic structure methods. Ab initio calculations of beta rely on quadratic response theory, which recasts the sum-over-all-states expression of beta into a closed-form expression by calculating a handful of first- and second-order response states; for resonantly enhanced beta, damped response theory is used. These response states are then used to construct second-order response reduced one-particle density matrices (1PDMs), which, upon visualization in terms of natural orbitals (NOs), facilitate a rigorous and black-box mapping of the underlying electronic structure with beta. We explain the interpretation of different components of the response 1PDMs and the corresponding NOs within both the undamped and damped response theory framework. We illustrate the utility of this new tool by deconstructing beta for cis-difluoroethene, para-nitroaniline, and hemibonded OH.+H2O complex, computed within the framework of coupled-cluster singles and doubles response theory, in terms of the underlying response 1PDMs and NOs for a range of frequencies.