# Artificial Photosynthesis: Is Computation Ready for the Challenge Ahead?
## 2 Light Harvesting
### QM/MM-Based Approaches
As results, the authors showed that, in order to reproduce accurate 2D maps, the pulse envelops must be included in the simulations. As result of this complex and highly accurate methodology, they were able to simulate several observed signals in the 2D ES maps, and indicate that there is a contribution form a dark state which was predicted by experimental observations but not yet observed by computations (Figure 4). Moreover, this approach shows that the integration of multiscale models and experiments is crucial to reveal the complex network of ET routes in LH2. Figure 3: Scheme of the procedure used to get transition dipole moments and transition densities from real-time propagation TD-DFT (P-TDDFT) calculations for multichromophore systems. Reprinted with permission from [53]. Copyright (2019) American Chemical Society ([[https://pubs.acs.org/doi/abs/10.1021/acs.jctc.9b00209](https://pubs.acs.org/doi/abs/10.1021/acs.jctc.9b00209)]([https://pubs.acs.org/doi/abs/10.1021/acs.jctc.9b00209](https://pubs.acs.org/doi/abs/10.1021/acs.jctc.9b00209)). Further permissions related to the material excerpted should be directed to the ACS). The same authors reported state-of-the-art methodologies in a subsequent study, considering multichromophoric aggregates of different LH complexes of different origins, in which different families of pigments, such as (bacterio)chlorophylls, carotenoids, and phychobilins, are involved (Figure 5) [68]. In this study the authors presented a common strategy to assess not only the different chemical nature of all the pigments, but also their spatial arrangement. In this computational approach, the strategy should be general enough to be applied to the variety of different pigment families, but at the same time should be tunable, to adapt to the peculiarity of each family. The real challenge faced in this work is the atomistic modeling of the whole photosynthetic light-harvesting complex, due to the very large dimensions, the strong interaction in between the pigments and the presence of the coupling within the different parts, which cannot be neglected. As the amount of pigment is significant and the degree of conjugation is high, the choice of QM methods to describe the chromophores is rather limited to TD-DFT methods. Yet, its bottlenecks (inaccurate description of CT states and impossibility to describe multiconfigurational character excitations) should always be remembered. As previously described, when considering atomistic models such as QM/MM, the environment is often considered either with the electrostatic or the polarizable embedding [69]. A different, more rigorous approach considers the use of the frozen density embedding scheme (FDE) [70], in which the system is divided into a number of fragments each described at the QM level, but only one of them is kept free to move, while all the other are frozen [71], thus completely eliminating the classical description of the environment. Although the computational cost of this approach is higher, the advantage is the ability of explicitly obtaining the electronic exchange and overlap terms, and not to add them to the Coulomb term of the Hamiltonian as commonly done [72]. With the presence of the protein affecting the coupling, different mechanisms should be considered [73], either through the effect of the transition densities of the dimers (implicit mechanism) or by screening of the Coulomb interaction (explicit mechanism), Figure 6.  
Figure 4: Comparison of simulated (**d**,**e**) and experimental (**f**) 2 DES pump-probe maps of Rps. acidophila at 300 K and waiting time \(t_{2}\) = 22 fs. The corresponding linear absorptions (blue) and pulse shapes (red), along with the relative experimental linear spectra (dashed line), are reported in (**a–c**), respectively. In the calculated maps the pulse shape was adapted to have the same overlap between pulses and absorption bands as the one reported in the experiments.