Ab initio calculations, also known as first-principles calculations, are computational methods used to predict the properties of molecules and materials based on quantum mechanics. These calculations can be employed to predict the excited state energy levels and lifetimes of organic molecules by solving the Schrödinger equation for the molecular system.To predict the excited state energy levels and lifetimes of organic molecules using ab initio calculations, the following steps are typically followed:1. Construct a molecular model: The molecular structure of the organic molecule is represented using a set of atomic coordinates and atomic types.2. Choose a basis set: A basis set is a mathematical representation of the atomic orbitals in the molecule. The choice of basis set affects the accuracy and computational cost of the calculations.3. Select an appropriate method: Various ab initio methods are available, such as Hartree-Fock HF , Configuration Interaction CI , Multi-Configuration Self-Consistent Field MCSCF , and Coupled Cluster CC methods. The choice of method depends on the desired accuracy and computational resources available.4. Perform the calculations: The chosen method is used to solve the Schrödinger equation for the molecular system, yielding the ground state and excited state energy levels.5. Calculate lifetimes: The lifetimes of the excited states can be estimated by calculating the transition probabilities between the excited states and lower energy states, which are related to the Einstein coefficients for spontaneous emission.Several factors affect the accuracy of ab initio predictions of excited state energy levels and lifetimes:1. Basis set: The choice of basis set has a significant impact on the accuracy of the calculations. Larger basis sets provide more accurate results but require more computational resources.2. Electron correlation: The inclusion of electron correlation effects is essential for accurate predictions of excited state properties. Methods that account for electron correlation, such as CI, MCSCF, and CC, generally provide better results than the simpler HF method.3. Molecular geometry: The accuracy of the predictions depends on the quality of the molecular geometry used in the calculations. Optimizing the geometry at the same level of theory as the excited state calculations can improve the results.4. Solvent effects: The presence of a solvent can significantly affect the excited state properties of organic molecules. Including solvent effects in the calculations, either implicitly or explicitly, can improve the accuracy of the predictions.5. Computational resources: The accuracy of ab initio calculations is often limited by the available computational resources. More accurate methods and larger basis sets require more memory and CPU time, which may not always be feasible for large organic molecules.