Nonlinear response theory plays a central role in theoretical materials chemistry. As such, applications of nonlinear response theory within time-dependent density functional theory (TDDFT) are increasingly important, from the calculation of nonlinear optical properties used to characterize complex materials to the excited-state properties and couplings needed to simulate non-adiabatic molecular dynamics (NAMD). However, several shortcomings of nonlinear response theories have recently been described, such as unphysical divergences in excited-state properties and inconsistent behaviors of multiphoton processes near one-photon resonances, sowing doubts as to how broadly applicable existing nonlinear response theories are. To examine this question at the root, I will discuss the foundations of response theory and connect the known failures of the nonlinear response properties to qualitative deficiencies in the higher-order response pole structure. I will illustrate the successes and failures of current approaches with electronic hyperpolarizability and two-photon absorption calculations of organic chromophores as well as with NAMD simulations of the first steps of water splitting on TiO<sub>2</sub> nanoparticle surfaces. Finally, I will close by discussing potential avenues for improvement.

Nonlinear properties from TDDFT: Trials and tribulations

253rd National Meeting (2017)

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