The absorption of light by a molecule or material occurs when a photon interacts with the electrons in the system. This interaction can cause an electron to be excited from its ground state to a higher energy state, leading to changes in the electronic structure of the molecule or material. This process is known as electronic excitation.When the electronic structure of a molecule changes, its chemical reactivity can also be affected. This is because the distribution of electrons in the molecule determines its chemical properties, such as bond strength, bond length, and reactivity towards other molecules. When an electron is excited to a higher energy state, it can weaken or strengthen existing bonds, create new bonds, or break existing ones. This can lead to the formation of new chemical species or the rearrangement of the molecular structure.Quantum chemistry methods can be used to study these changes in electronic structure and chemical reactivity. These methods involve solving the Schrödinger equation for the molecular system, which describes the behavior of electrons in the molecule. By solving this equation, we can obtain information about the energy levels of the electrons and the distribution of electron density in the molecule.There are several quantum chemistry methods that can be used to study the absorption of light and its effects on electronic structure and chemical reactivity:1. Time-dependent density functional theory TD-DFT : This method is an extension of density functional theory DFT that allows for the study of excited states and their properties. TD-DFT can be used to calculate the absorption spectra of molecules and materials, as well as the changes in electronic structure upon excitation.2. Configuration interaction CI and multi-configurational self-consistent field MCSCF methods: These methods involve the construction of wave functions that are linear combinations of multiple electronic configurations. They can be used to study excited states and their properties, as well as the changes in electronic structure and chemical reactivity upon excitation.3. Time-dependent wave function methods: These methods involve solving the time-dependent Schrödinger equation for the molecular system, which allows for the study of the dynamics of electronic excitation and relaxation processes. This can provide insights into the changes in electronic structure and chemical reactivity that occur upon absorption of light.By using these quantum chemistry methods, chemists can gain a deeper understanding of the processes that occur when a molecule or material absorbs light, and how these processes affect the electronic structure and chemical reactivity of the system. This knowledge can be applied to the design of new materials with specific optical properties or the development of novel photochemical reactions.