# Artificial Photosynthesis: Is Computation Ready for the Challenge Ahead?
## 2 Light Harvesting  
A vast amount of pigment-protein complex (PPC) crystal structures have been solved in the last decades, including highly symmetric systems such as light-harvesting complexes in purple bacteria or less ordered ones like the major light-harvesting complexes (LHCI and LHCII) of plants. PPCs can be considered as models for light-harvesting since both the capture of sunlight by antenna complexes (the initial step of photosynthesis) and the following transfer of the absorbed energy to the reaction centers (RCs, where charge separation is realized) are achieved with extremely high efficiencies. The mechanisms involved in the electron transfer (ET) and electronic energy transfer (EET) in natural PPCs are known [19; 20; 21; 22; 23; 24; 25], and there is no theoretical obstacle in building its artificial analogues. Yet, this is still difficult to obtain, particularly regarding efficiencies and stabilities required for large-scale manufacturing. It is in here that computation can be considered as optimal method to assess these challenges. In fact, real advantages can be expected if accurate molecular-based theories could be transformed into efficient computational protocols able of simulating in an accurate and comparable way natural PPCs and artificial systems. Quantum chemistry is here the method of choice due to its versatility and extensibility towards different spatio-temporal scales, especially when combined with MD simulations. The \"holy grail\" of such theoretical approach would be to be able to perform computation of the light-harvesting process at the QM level of theory, followed by the quantum dynamic simulations of the excited states to follow in time the photoinduced process [26; 27; 28; 29; 30]. This approach would give access to the EET dynamics and will also allow an extremely accurate simulations of the optical spectra which, in turn, dictate the spectral cross-section for light absorption, enabling a direct comparison with the experimental knowledge of natural photosynthetic complexes. Unfortunately, the complexity of LH systems and the size of the network of interacting chromophores have so-far precluded the direct application of this approach. Despite this drawback, computation can still be efficiently used with QM/MM. This method combines the description of the chromophore at the QM level and the classical description of the environment. The success of this method rely in the explicit description of the interactions between the two parts. The classical part is generally divided into two families, depending on the type of description considered for the environment, namely as a continuum or with explicit account of the atomic degree of freedoms by using QM  
Figure 1: Multiscale approach for the study of photosynthetic interfaces. Different spatio-temporal resolution are reported. While the system size increases, longer simulation times are required, at the expenses of a decrease in accuracy. Adapted from [14], [PERSON], 2020. Abbreviations: Quantum mechanics (QM), QM/molecular mechanics (MM), and molecular dynamic (MD). forcefields. The QM part can be also be described in its electronic and nuclear terms due to the presence of the classical part. The total Hamiltonian becomes:  
\[H=H_{\text{QM}}+H_{\text{env}}+H_{\text{int}} \tag{1}\]  
where \(H_{\text{QM}}\) is the Hamiltonian of the isolated chromophore (in vacuum), \(H_{\text{env}}\) of the rest of the system expressed at the classical level and \(H_{\text{int}}\) of the interactions between the two. In both QM and classical descriptions the degrees of freedom of the environment are only marginally important, while in the continuum solvation model (in which the atomistic description disappears) this term disappears. \(H_{\text{env}}\) is maintained for QM/MM approach as a constant term to the total energy. The most common formulation of the QM/MM approach considered the MM part described as a set of fixed point charges placed at the atoms positions of the environments.