# Artificial Photosynthesis: Is Computation Ready for the Challenge Ahead?
## 2 Light Harvesting
It is thus evident that to accurately describe light-harvesting processes in both artificial and natural systems, simplification should be taken with care, as completely neglecting the environment will lead to inaccurate descriptions of the processes of light harvesting and EET. Different strategies have been developed over the years to integrate the environment within a quantum chemical description of an embedded molecule. The most computationally efficient and straightforward approach is to consider the environment as an infinite continuum dielectric (i.e., polarizable continuum model, PCM). By adding a surface charge distribution to model an induced polarization, mutual influence of the environment over the QM part is obtained [31]. The definition of the cavity in which the QM part is immersed becomes the key parameter for this method. This approximation results in very good ratio between accuracy and computational cost, explaining its popularity. In cases when specific interactions between the QM part and selected atoms of the environment are important (such as van der Waals, electrostatic, and hydrogen bonding) this approach fails, and an atomistic description of the environment must be introduced. In such cases the method of choice is QM/MM, in which the environment is described through molecular mechanics (MM) force fields [32]. This lead to electrostatic interactions only between the QM and MM parts, in which the MM charges represent the molecular electrostatic properties of the environment. Yet, more sophisticated methods can be considered to describe the MM part, namely with higher multipole moments, such as dipoles, in which the polarization of the environment is additionally taken into account. The widely used electrostatic embedding scheme considers the inclusion of the effect of the MM atoms in the QM Hamiltonian, having the same form of the Coulomb term [33]. The interaction term of the Hamiltonian can be thus defined as:  
\[H_{\text{int}}=\sum_{i}q_{i}V_{\text{QM}}(r_{i}) \tag{2}\]  
where \(q_{i}\) are the partial charges at position \(r_{i}\) and \(V_{\text{QM}}(r_{i})\) is the electrostatic potential operator due to the QM charge density. In this scheme, the electronic structure of the QM part is modified by the presence of the MM charges representing the environment and its polarized by it. Yet, the opposite scenario (MM term polarized by the QM one) is not present and this is the main limitation of the electrostatic embedding, as the explicit response of the environment might be of extreme importance when polar environment are present or when excitation processes are considered. Despite this main drawback, the electrostatic embedding is still widely applied, especially because of the short computational time required, when compared to more sophisticated descriptions. Nevertheless, recently different improvements started to be used also for the study of LH systems. A step forward in a more accurate description of the environment effect on the QM properties is the polarizable embedding, in which mutual polarization between the classical MM and the QM parts is explicitly included. In particular, we refer here to the induced dipole model in which polarizabilities are introduced. In this way, dipoles are present on both QM and MM parts and interact with each other, leading to a self-consistent procedure to solve this QM/MM interactions. As consequence, an additional parameter is added to the Hamiltonian in Equation (2), namely the atomic polarizabilities, defined as a 3 \(\times\) 3 tensor [16]. The main drawback of this last method is the high computational cost to pay for accounting of these interactions, for which the computation of the polarizabilities term is the most computationally time consuming. A recent development consists on the merging of MM and continuum approaches to give fully polarizable QM/MM, in which in addition to the polarizable model described above an external polarizable continuum layer coupled to the former is added.