Why Computational Photochemistry Is Challenging and Will Probably Remain So: A Quantum Chemist's Perspective.

Compared to ground-state chemistry, the theoretical description of molecular photochemistry, that is, chemical processes in electronically excited states or electronic spectra, is fundamentally more complex. Even for relatively small molecules, several open decay channels have to be considered at the same time, and thus several electronically excited states with different electronic structures need to be computed. Furthermore, the required accuracy of the employed electronic structure theory increases with the size of the studied molecule, even for a qualitative investigation of photochemical processes. They critically depend on the correct relative energies of the electronic states and the details of the underlying potential energy surfaces (PES), and with increasing molecular size, the excitation energies become smaller and smaller, and more and more electronic states lie within the error margins of most common electronic structure methods. This imposes clear size limits for tractable molecular systems and introduces an early and fast transition from highly accurate calculations to wild guessing. Computational chemistry always lives with the lazy compromise between accuracy and computational effort, and this is particularly true for computational photochemistry, as will be discussed within this perspective from the point of view of electronic structure theory.