The calculation of excited electronic states plays a crucial role in predicting optical properties such as absorption and emission spectra in a particular chemical compound. This is because the interaction of light with matter, which leads to the absorption or emission of photons, is directly related to the electronic structure of the compound.When a chemical compound absorbs a photon, an electron is promoted from a lower-energy ground state to a higher-energy excited state. This process is called electronic excitation. The energy difference between the ground state and the excited state corresponds to the energy of the absorbed photon, which determines the wavelength of light absorbed. This information can be used to predict the absorption spectrum of the compound.Similarly, when an electron in an excited state returns to a lower-energy state, it emits a photon with energy equal to the energy difference between the two states. This process is called electronic emission, and the emitted photon's energy determines the wavelength of light emitted, which can be used to predict the emission spectrum of the compound.To calculate the excited electronic states and predict the optical properties of a chemical compound, quantum chemistry methods are employed. These methods involve solving the Schrödinger equation for the compound's electrons and nuclei, which provides information about the energy levels and wavefunctions of the electrons.There are various computational methods to calculate excited electronic states, such as:1. Time-dependent density functional theory TD-DFT : This method is widely used for calculating excited states and optical properties of molecules. It is based on the density functional theory DFT framework, which calculates the ground state electronic structure, and extends it to include time-dependent phenomena like electronic excitations.2. Configuration interaction CI and multi-configurational self-consistent field MCSCF methods: These methods are based on the wavefunction approach and provide accurate excited state energies and wavefunctions by considering multiple electronic configurations.3. Coupled-cluster CC methods: These methods are also based on the wavefunction approach and provide highly accurate excited state energies by considering the correlated motion of electrons.By calculating the excited electronic states using these methods, one can obtain the energy differences between the ground and excited states, which can be used to predict the absorption and emission spectra of the chemical compound. Additionally, the calculated wavefunctions can be used to compute other optical properties such as oscillator strengths, transition dipole moments, and radiative lifetimes, which provide further insights into the compound's photophysical behavior.