# Artificial Photosynthesis: Is Computation Ready for the Challenge Ahead?
## 2 Light Harvesting
### QM/MM-Based Approaches
These features open the way to the study of large biological systems such as LHC, while explicitly accounting for the environment and treating it at the same atomistic level of theory [61]. Within this approach, the transition dipole moments and transition densities of the individual pigments can be obtained, permitting the computation of a broad excited states' energy range without neglecting the effects induced by the environment. [PERSON] et al. [53] applied P-TDDFT to LHC systems, considering all the photoactive chromophores at the same ab-initio level of theory, following the procedure reported in Figure 3. Despite the inherent problem of TDDFT in properly describe CT states, the authors obtained accurate results especially for weakly interacting (large) systems by explicitly accounting for quantum mechanical environment effects. Moreover, they introduced a local density analysis to determine the transition properties of multichromophore systems. In detail, this procedure permit to extract accurate transition dipole moments and densities for an individual chromophore, while taking into account the different effects of the environment surrounding it, without any approximation (the whole system is considered at the same ab-initio level of theory). Thanks to this approach the excitonic coupling, a key parameter for EET process, can be accurately computed, as well as the exciton transfer. Figure 2: Structure of photosystem I (PSI, (**a**)) and photosystem II (PSII, (**b**)) from pdb entry 1 JB0 and 3 WU2, respectively. Trimeric light-harvesting complex (LHC II) complex from pdb entry 2 BHW; the different chains are indicated by different colors (**c**); chlorophyll residues in chain A, in which the Chl residues are colored in yellow and red tones (**d**). Reproduced from [47] with permission from The Royal Society of Chemistry, 2011. Representation of the BRC of Rhodobacter sphaeroides based on X-ray study (**e**). The core pigment units are the P, and the pairs of BChl, and BPhe, shown in the central part along with representation of their neighbouring pigments that form the distinct effective dielectric for the two branches. Reprinted with permission from [50]. Copyright (2019) American Chemical Society. Recently, due to the enhancement in computational power, the QM/MM approach has been strongly revaluated to not only consider polarizable embedding in the MM part, but also to improve the QM description by going beyond the mono-configurational picture which is behind DFT methods. [PERSON] et al. developed a hybrid QM/MM model which accounts for the effect of the environment when studying LH2 [62]. In particular, they consider two levels of QM theory: TD-DFT and RASSCF/RASPT2 methodology [63; 64] with different Active Spaces and State Average settings in the different spectral regions. When TD-DFT is considered, both electrostatic and polarizable embedding have been considered for the MM part, while for the more computational expensive RAS calculations the charges were considered within the electrostatic embedding. For the LH2 system, in which pigments in the Qy region are strongly coupled, the authors considered the Redfield regime for the disordered exciton model [43; 65], while for the Car-Ox region the Forster theory has been considered, as the reorganization energy is large in this region and the energy gap between Car and Ox is considerable. In addition, the 2D excited state pump-probe spectra were simulated with the Sum Over States (SOS) approach [66; 67] using the transition dipole moments, couplings and site energy as obtained from the QM part (either TD-DFT or RAS). With this novel approach, the authors were able to simulate both linear and 2D spectra over the Qy and Car-Ox regions, without relying on simplified descriptions of the environment or approximated QM methods. Both single and multireference QM methods have been used together with electrostatic and polarizable MM embeddings in order to properly describe the whole protein.