# Artificial Photosynthesis: Is Computation Ready for the Challenge Ahead?
## 2 Light Harvesting
### QM/MM-Based Approaches
Panel (**e**) refers to a calculated map where Car \(S_{2}\) site energies and \(Q_{\mathrm{c}}\) transition dipoles have been scaled to match the position and the intensities measured in the linear spectrum. Simulated maps are normalized to their maximum. By convention, bleach and stimulated emission contributions appear as positive (red) signals, excited state absorption appears as negative (blue) peaks. Full (referring to Cars) and empty (referring to BChls) geometric symbols are used to indicate correlations between calculated and measured signals. Reprinted with permission from [62]. Copyright (2017) American Chemical Society. Considering all these interactions explicitly, without introducing any approximation related to the methods used, the authors were able to reveal the delicate interplay of the different mechanisms over all the components on LH systems. In this impressive state-of-the-art work, the authors reported novel strategies suitable for simulating the dynamic of real systems with tens of thousands of atoms at a very high level of accuracy, taking into account all the different scales of the components in a very coherent way. This study really pushed the boundary of computation beyond the state-of-the-art, opening novel, exciting perspectives on the integration of new theoretical models and algorithms, as well as new tools for the analyses. [PERSON] et al. [74] applied a different QM/MM approach to study the Fenna\(-\)Matthews\(-\)Olson (FMO) pigment\(-\)protein complex. FMO is one of the most studied LH complex, present in green sulfur bacteria which primary function is to transfer the excitation energy form the antenna complex to the reaction center, where the charge transfer process is initiated. FMO is a trimer in which each subunit possess eight bacteriochlorophyll a (BChI a) chromophores. Its structural simplicity and the complex excitonic structure makes FMO an ideal system for testing novel computational techniques. The authors of this study reported a comprehensive multiscale approach to reproduce the absorption and circular dichroism spectra without any input from experiment, apart the crystal structure of the complex itself, in a \"single-blind\" like approach. They showed that using the QM/MM together with the effective fragment potential (EFP) model to treat the environment while including polarization is the key to properly assess the excitonic structure. To reach this conclusion, the authors developed a computational protocol in which the FMO complex is fist immersed in water and equilibrated via several MD simulations, then constrained geometry optimization at the QM/MM level are computed to remove inaccuracies caused by inaccurate force field description. Next, excited state calculations with the polarizable embedding QM/EFP scheme [75; 76; 77] were performed,  
Figure 5: Examples of multichromophoric aggregates as found in different LH complexes. Starting from the left upper corner we see the major LH complex of plants LHCII (here represented in its monomeric form), the LH2 from _Rhodoblastus acidophilus_ (purple non-sulfur bacteria), the phycoerythrin 545 (PE545) antenna of _Rhodomonas_ sp. (marine algae), the peridinin–chlorophyll a–protein (PCP) from _Amphidinium carteae_ (oceanic plankton) and the Fenna–Matthews–Olson (FMO) complex of _Prosthecochloris aestuarii_ (green sulfur bacteria). In the central ellipse a schematic representation of the different types of pigments is reported; namely (bacterio)chlorophylls, carotenoids and bilins. Reprinted with permission from [68]. Copyright (2020) Elsevier. to obtain site energies and transition charges (used to compute the electronic couplings) of the pigments. We would like to remind here that the described methodology is still debated, especially the constrained optimization, as it has been reported that it removes the benefit of performing MD simulations, and has been suggested that a thorough force field parameterization to avoid this step should instead be considered.