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The interplay of electronic and nuclear motions upon molecular photoexcitation challenges the most advanced computational techniques on two fronts: the electronic structure and the quantum nuclear dynamics [1-5]. Continuous advancements in computational methods to describe photoinduced molecular processes are needed to deepen our understanding of photochemical reactions and to accompany recent developments in experimental techniques, which allow to trace the time evolution of photoexcited molecules with unprecedented time resolution [6,7].

The past 40 years have seen large developments of computational methods for nonadiabatic dynamics, which describe in different ways (1) the coupling of electronic/nuclear motion in nonadiabatic conditions and (2) the nuclear dynamics itself, employing techniques of various nature ranging from quantum to classical [8-10]. Some of these methods can treat various different dynamical aspects, like different spin multiplicities [11-13], the effect of solvation [14-17], or including radiation fields [18-21]. Finding a suited nonadiabatic dynamics method for a problem at hand among the plethora of different techniques with multiple flavors is not a trivial task, which is exacerbated by the lack of systematic comparisons and rigorous testing on realistic molecular systems [22].

Defining standards and benchmarking are important in all branches of science to evaluate reliability and reproducibility of scientific measurements. The nonadiabatic dynamics community has, over the past decade, reached a general consensus that reliable benchmark studies on realistic molecular systems are missing although strongly needed [23]. The lack of relevant benchmarks in the field is a pressing issue which urges for action. While low-dimensional test systems have already been established [24, 25], benchmarks of methods, algorithms and implementations on realistic molecular systems are limited or completely absent. A wide collaboration within the community on this large project devoted to create standards and propose benchmarks for nonadiabatic dynamics techniques is needed. The aim of the proposed workshop is to actively start this collaboration and define the first set of problems to be tackled.

Related areas of research in theoretical chemistry/physics benefited significantly from introducing common standards. There have been highly successful projects, such as Delta-Project creating a benchmark set in solid-state DFT [26], Thiel’s benchmark set for electronic-structure theory [27], HyDRA Challenge for computational spectroscopy (https://qmbench.net/challenges/hydra/hydra), Fe-MAN challenge for computational microkinetics (https://qmbench.net/challenges/feman/feman). While these actions serve as inspiring examples, we note that benchmarking nonadiabatic dynamics is more complex, as compared with electronic structure theories that usually deal with calculating energies and related spectral features as obvious observables.

Comparing different nonadiabatic dynamics methods is rather "multidimensional", since numerous factors need to be taken into account. Primarily, what is the basis for the comparisons? Should a reference be an experiment or the most accurate theoretical results? Which observable property is the fairest to be considered as the "result" of the dynamics? Examples might be spectroscopic signals or quantum yields, rather than diabatic or adiabatic electronic populations which are not physical observables. Another point of discussion will concern the nature of the methodologies to compare. Will quantum nuclear effects be important in the proposed set of benchmark problems? Entangled with trajectory-based techniques is also the issue of the preparation of the system. Can the methods handle initialization as a coherent superposition of electronic states? How should initial conditions be sampled? Overall, we aim to bring members from the nonadiabatic dynamics community together to tackle these pressing questions.

Workshop's details

The aim of our workshop is to work towards building a consensual and widely accepted benchmark set for nonadiabatic dynamics methods. With an alternative format, we want to encourage open discussions on how we can compare and evaluate the strengths and limitations of different nonadiabatic dynamics methods and to work towards establishing a protocol to benchmark existing and novel methods and implementations. We invite speakers to present problems specifically related to benchmarking nonadiabatic dynamics methods, presenting concrete molecular systems that could be suitable for trajectory-based methods, Gaussian wavepacket dynamics, quantum molecular dynamics etc. as well as observables that could be used for the evaluation and comparisons. The workshop will be split in two parts. First, the active discussions about benchmarking problems, molecular systems and relevant observables, and, second, the hands-on testing, where the participants will examine the suggested systems with their own methods and codes.

The final aim of the workshop is to establish a consensus on a general direction for the creation of molecular benchmarks as well as to gather a team of people dedicated to contribute to a large collaborative effort of examining nonadiabatic dynamics methods and implementations and creating a standardized benchmark set. On the final day, a large discussion session will be held to reach a preliminary decision on a set of molecular benchmarks. In addition, there will be a formation of working groups that will work on making this benchmark set accessible, and will start testing their own codes and methods on said set.

We invite young researchers to contribute with flash presentations, where they shortly introduce a concrete problem from their research, where they encountered a challenge for nonadiabatic dynamics methods.