The task of optimizing four materials for tandem cells is challenging. To simplify it, the top donor/acceptor pairs determined in the previous step are used as one subcell in a tandem structure. The new task is then to find new acceptor and donor combinations that 1) maximize absorption, 2) minimize spectral overlap, and 3) have aligned energy levels. During the re-population of each generation, each individual is checked to ensure energy level alignment between the acceptor and donor (acceptor's frontier orbitals are lower than the donor's). The fitness function is calculated as Score = Absorption -Overlap. Absorption is measured as the integral of the summed oscillator strengths ( f ) of the four materials of a simulated Gaussian blur spectrum between 0.5 and 10 eV, seen in eqn. 1.