Photochemistry is central in our day-to-day life. Not surprisingly then, it has long intrigued mankind, as the underlying light matter interaction is at the origin of many vital processes such as photosynthesis and vision for example. Thus, understanding photochemical processes is of critical importance in order to comprehend the world around us. The relevance of photochemistry also lies in the various technological applications that have been developed over the years making use of the specific chemistry and properties initiated by the population of electronically excited states upon irradiation of molecular systems or materials.[1] Prominent examples are sensors,[2] data storage,[3] photovoltaics,[4] light-emitting diodes,[4,5] and phototherapy.[6]
In the past three decades or so, computational photochemistry has gained considerable credit as a tool to investigate photochemical reaction mechanisms in organic, inorganic and even biological chromophores.[7] This reputation has been gained thanks to the concomitant growth of computational power and theoretical developments in the field of quantum chemistry. These advances allow peering beyond the traditional interpretations of photochemistry focused on vertical excitations at the Franck–Condon geometry. The exploration of other regions of the complex multidimensional potential energy surfaces is becoming routine in relatively small molecular systems, and the synergy between accurate and global static calculations and either quantum or semiclassical nonadiabatic molecular dynamics simulations has allowed major breakthroughs in the understanding of photochemical and photophysical processes.
While many computational photochemical studies have been devoted to organic photoactive molecules, theoretical investigations of the photochemistry of inorganic systems such as transition metal complexes are still relatively scarce. Among the possible reasons one can cite the difficulty i) to describe accurately electronic excited states in coordination compounds, ii) to identify the excited states that are involved in the photochemical process due to the high density of electronic states present, iii) to investigate potential energy surfaces coupled by interstate and spin-orbit couplings, iv) to determine photochemical pathways evolving on these potential energy surfaces, and v) to simulate the photodynamics of such complex systems. While all these challenges are also present to some extent in organic computational photochemistry, they are in practice much more difficult to solve in metal complexes. For example, in terms of electronic structure methods, describing photochemical paths in metal complexes usually require the use of quantum chemical approaches that take into account both static and dynamic electron correlations, making the complete active space self-consistent field (CASSCF) method so often used in organic computational photochemistry[8] inadequate. However, methods describing accurately the electron correlation often lack the energy gradients necessary to explore the potential energy surfaces, the photochemical paths and to simulate the photodynamics. It is therefore necessary to find a compromise and, despite their limitations, density functional theory (DFT)-based methods have often been used to explore photochemical properties of metal complexes,[9–15] while accurate wavefunction-based methods represent a formidable challenge.[12,16–19]

The workshop aims to gather theoreticians working either in the field of computational photochemistry or in methodological developments of electronic structure methods to focus on the ways theory can address the various challenges one is confronted with when studying photochemical processes, with special emphasis on the photochemistry of metal complexes. A few renowned experimentalists working in the field and who have strong connections with computational photochemists have also been invited for enriching the discussions with their perspectives and identifying critical challenges in the field.

The talks and discussions will focus on the following critical topics:

• Electronic structure calculations: quantum chemical approaches adapted to the calculations of coupled (by interstate and spin-orbit couplings) excited states in metal complexes and allowing for static and possibly dynamic investigations of the photochemical paths. DFT-based methods: TD-DFT, multiconfigurational DFT. Wavefunction-based methods: multiconfigurational second-order perturbation theory (CASPT2, NEVPT2, XMS-CASPT2, …), density matrix renormalization group (DMRG), DMRG-CASPT2, stochastic ab initio methods. Development of analytic energy gradients.

• Electronic structure analysis: density-based indexes, transition density matrices, natural transition orbitals, charge transfer numbers.

• Static investigation of photochemical paths: minimum energy paths, nudged elastic band, linear interpolation methods, image dependent pair potential.

• Non-adiabatic excited state dynamics: wavepacket dynamics using vibronic coupling models, on-the-fly trajectory surface hopping dynamics, multiscale simulations to include environment effects.

• Interplay between theory and experiments: experimental observables versus computed data, mechanistic photochemistry vs quantitative description of photochemical processes.