The calculation of electronic excited states and optical properties of a molecule in quantum chemistry is a complex task that requires the use of appropriate computational methods and basis sets. The choice of these factors can significantly influence the accuracy and reliability of the results obtained. Here, we will discuss how the choice of computational method and basis set can affect the calculation of electronic excited states and optical properties.1. Computational methods:There are several computational methods available in quantum chemistry to calculate electronic excited states and optical properties, ranging from less accurate and less computationally demanding methods like semi-empirical and Hartree-Fock HF methods, to more accurate and computationally demanding methods like Configuration Interaction CI , Multi-Configurational Self-Consistent Field MCSCF , and Coupled Cluster CC methods. The choice of the method depends on the size of the molecule, the nature of the excited states, and the required accuracy of the results.a Semi-empirical and Hartree-Fock methods: These methods are less accurate but computationally less demanding, making them suitable for large molecules or preliminary calculations. However, they often fail to describe the excited states and optical properties accurately, especially for systems with significant electron correlation effects.b Time-Dependent Density Functional Theory TD-DFT : TD-DFT is a widely used method for calculating excited states and optical properties due to its balance between accuracy and computational cost. However, the accuracy of TD-DFT depends on the choice of the exchange-correlation functional, and it may not be suitable for systems with strong electron correlation or multi-reference character.c Configuration Interaction, Multi-Configurational Self-Consistent Field, and Coupled Cluster methods: These methods provide more accurate results for excited states and optical properties, especially for systems with strong electron correlation or multi-reference character. However, they are computationally more demanding and may not be feasible for large molecules.2. Basis sets:The choice of basis set is crucial for obtaining accurate results in quantum chemistry calculations. Basis sets are used to represent the molecular orbitals and can vary in size and complexity. The choice of basis set affects the accuracy of the calculated excited states and optical properties, as well as the computational cost.a Minimal basis sets: These basis sets, such as STO-3G, use the smallest number of basis functions to represent the molecular orbitals. They are computationally less demanding but often provide less accurate results for excited states and optical properties.b Split-valence basis sets: These basis sets, such as 6-31G or 6-311G, use a larger number of basis functions to represent the molecular orbitals, providing more accurate results for excited states and optical properties. However, they are computationally more demanding than minimal basis sets.c Polarization and diffuse functions: Adding polarization functions e.g., 6-31G* or 6-311G* and diffuse functions e.g., 6-31+G* or 6-311++G** to the basis set can improve the accuracy of excited state and optical property calculations, especially for systems with charge-transfer excitations or large electron density changes upon excitation. However, these additions increase the computational cost.d Correlation-consistent basis sets: These basis sets, such as cc-pVDZ, cc-pVTZ, and cc-pVQZ, are specifically designed for use with correlated methods like CI, MCSCF, and CC. They provide more accurate results for excited states and optical properties but are computationally more demanding.In conclusion, the choice of computational method and basis set in quantum chemistry calculations significantly affects the accuracy and reliability of electronic excited states and optical property predictions. The choice should be based on the size of the molecule, the nature of the excited states, the required accuracy, and the available computational resources.