Quadratic response properties from TDDFT: trials and tribulations

Nonlinear response theory is an increasingly important theoretical tool used to compute, for example, nonlinear optical properties needed to characterize complex materials or electronic couplings between excited states in time-dependent density functional theory (TDDFT). I will present an efficient implementation of the TDDFT quadratic response function, including the static and dynamic hyperpolarizability, two-photon absorption amplitudes, and excited-state absorption amplitudes. The successes and failures (with respect to the quadratic response function's recently detailed incorrect pole structure) of existing nonlinear response methods will be sketched out by computing the dynamic hyperpolarizability of several steroisomers of octupolar calix[4]arenes, the two-photon absorption spectra of a series of twisted conjugated porphyrins, and the excited-state absorption spectra during excimer formation in perylene diimide dimers. I will close with guidelines for practitioners on avoiding spurious resonances in applications, and by discussing possible remedies.