Light triggers many important chemical reactions. These include photosynthesis, which converts sunlight to chemical energy and powers most life on earth, human vision, which as its first step uses a photochemical reaction in our retinas to detect light, and technologies such as photodynamic therapies for cancer, molecular photonics, photovoltaics, and organic LEDs. In this area of research, computational modelling is crucial to interpret experiment, analyse data, and provide a broad conceptual framework that provides true insight. Yet, computational modelling remains tremendously challenging, in essence because the photon (‘light-particle’) absorbed by a molecule in a photochemical process carries a large amount of energy, which forces the electrons and nuclei into complex coupled motion that must be modelled using quantum mechanics, making computations exponentially more difficult. The solution to this conundrum is to combine specialised techniques that calculate the motion of electrons with new quantum dynamics methods that allow us to simulate the motion of the nuclei. Doing so will allow us to create the most accurate simulations of photochemical reactions to date. We will use these simulations to examine a new light-driven chemical reaction in the green-house gas CO2, that might have great utility for CO2 reduction technologies, and resolve a controversy regarding the photochemical ring-opening reaction of the 1,3-cyclohexadiene molecule. Our ultimate goal is to push simulations to such accuracy that we can use computers to design new molecules with specific properties for use in e.g. molecular photonics, organic LEDs or photovoltaics.