Excited state relaxation in organic molecules involves the dissipation of excess energy after the molecule has been excited to a higher electronic state, typically by the absorption of a photon. Ab initio calculations, which are based on quantum mechanics and do not rely on empirical data, can be used to study these relaxation processes. The mechanisms of excited state relaxation can be divided into several categories, including internal conversion IC , intersystem crossing ISC , fluorescence, and phosphorescence.1. Internal Conversion IC : This is a non-radiative process where the molecule relaxes from an excited electronic state to a lower electronic state within the same spin multiplicity e.g., from S1 to S0 . The excess energy is converted into vibrational energy, which is then dissipated as heat. The rate of IC depends on the energy gap between the states and the degree of vibrational coupling.2. Intersystem Crossing ISC : This is another non-radiative process, but it involves a change in the spin multiplicity of the molecule e.g., from S1 to T1 . The rate of ISC depends on the energy gap between the states, the degree of spin-orbit coupling, and the presence of heavy atoms that can enhance the coupling.3. Fluorescence: This is a radiative process where the molecule emits a photon as it relaxes from an excited singlet state e.g., S1 to the ground state S0 . The rate of fluorescence depends on the radiative lifetime of the excited state and the degree of vibrational coupling between the states.4. Phosphorescence: This is another radiative process, but it involves the emission of a photon as the molecule relaxes from an excited triplet state e.g., T1 to the ground state S0 . The rate of phosphorescence depends on the radiative lifetime of the triplet state and the degree of spin-orbit coupling.The relaxation pathway can change based on the electronic states involved and the solvent environment. For example, the solvent can affect the energy levels of the electronic states, leading to changes in the energy gaps and the rates of IC, ISC, fluorescence, and phosphorescence. Additionally, the solvent can influence the vibrational and spin-orbit couplings, which can also impact the relaxation pathways.To study these effects using ab initio calculations, one can perform calculations on the isolated molecule and the molecule-solvent complex. By comparing the results, one can gain insights into the role of the solvent in the excited state relaxation processes. Time-dependent density functional theory TD-DFT and coupled-cluster CC methods are commonly used for these calculations, as they can accurately describe the electronic structure and excited state properties of the molecule.