Generally speaking, photosynthesis as one of the few well known biological processes governed by quantum mechanics should be modeled using ab initio quantum mechanical simulations.
The problem is that such simulations are extremely computationally expensive. Hence, the next feasible approach would be molecular dynamical simulations with specially adjusted potentials that will, hopefully, capture the most critical QM processes relevant to photosynthesis.
Those simulations will still be computationally expensive but by orders of magnitudes less than the possible ab initio QM simulations.
I highly recommend starting by studying known QM approaches and all that is known about photosynthesis prior to designing any model.
My objective is to choose a tree and model it photosynthesis processes so as to approximate the average quantity of CO2 absorb during an average day by that tree