Filipp U. Furche, University of California, Irvine (Principal Investigator)

Electronically excited states play a pivotal role in the capture, transformation, storage, and emission of radiative energy. Despite much
recent progress for small model systems, excited state properties and dynamics in many larger molecular devices and materials have remained elusive for experiment and theory. Nonadiabatic transitions between electronic states turn radiative energy into heat and are particularly difficult to control. This project aims to develop new electronic structure and nonadiabatic molecular dynamics (NAMD) methods to simulate, predict, and ultimately control the flow and transformation of excitation energy in a wide-range of light-driven molecular devices such as dyes, photocatalysts, and fluorescence emitters.

With previous DOE support, we devised, built, and deployed the first general-purpose NAMD implementation using time-dependent hybrid density functional theory (TDDFT) during the past five years. This enabled the identification of nonadiabatic effects in acetaldehyde photodissociation and the discovery of excited-state photocatalytic reactivity in TiO₂ nanoclusters. However, our work also revealed spectacular unphysical divergences in conventional time-dependent response theory underlying most present-day excited-state computations. Here we propose an orbital-optimized ensemble TDDFT (oe-TDDFT) approach to address key remaining limitations in excited-state methodology: (i) oe-TDDFT obviates the need for divergent orbital-relaxation contributions in response theory. (ii) Based on experience with Δ-SCF theory and Ziegler's constricted variational methods, we hypothesize that non-linear orbital optimization will reduce
self-interaction error without the need of constructing new functionals. (iii) oe-TDDFT is designed to yield conical intersections
between all states with non-zero ensemble weight with correct dimensionality. The computational cost of the proposed oe-TDDFT method
is comparable to conventional TDDFT methods and should permit resource-efficient NAMD simulations for 50-100 atom systems. We
further propose to develop a consistent approach to tackle fast intersystem crossing in NAMD simulations by including spin-orbit coupling into our methods.

We will assess these methods by by comparison to gas-phase spectroscopy, photodissociation experiments, and accurate calculations. Our previous work on TiO₂ photocatalysis highlighted the importance of exciton localization and stabilization of the photohole by solvation. We will illustrate the use of our new methodology for photocatalyst development by exploring synthetically functionalized TiO₂ nanoparticles with organic ligands, and we will investigate the effects of particle size, shape, and defects on photocatalytic conversion efficiency.