The excited state chemistry of benzene involves the study of its behavior when its electrons are promoted to higher energy levels, typically through the absorption of light. The mechanism of this process can be understood through the concept of electronic transitions and the potential energy surfaces PES of the molecule.In benzene, the most common excited state is the singlet excited state S1 , which is formed when an electron from the highest occupied molecular orbital HOMO is promoted to the lowest unoccupied molecular orbital LUMO . This transition is typically denoted as * transition, as the HOMO and LUMO are both orbitals. Upon excitation, the molecule undergoes structural changes, which can lead to various photochemical reactions, such as isomerization, dissociation, or electron transfer.To optimize the excited state chemistry of benzene through ab initio calculations, several steps can be followed:1. Choose an appropriate level of theory: Select a suitable quantum mechanical method to model the electronic structure of benzene. Common methods include Hartree-Fock HF , density functional theory DFT , and post-Hartree-Fock methods like configuration interaction CI and coupled-cluster CC theory. The choice of method depends on the desired accuracy and computational cost.2. Perform geometry optimization: Optimize the ground state S0 geometry of benzene using the chosen level of theory. This will provide the equilibrium structure and the minimum energy of the molecule.3. Calculate excited state energies and transition properties: Perform a single-point calculation on the optimized ground state geometry to obtain the excited state energies, oscillator strengths, and other relevant properties. This can be done using methods like time-dependent DFT TD-DFT , equation-of-motion CC EOM-CC , or complete active space self-consistent field CASSCF .4. Analyze the potential energy surfaces: Construct the PES for the ground and excited states of benzene. This can be done by varying the molecular geometry along specific reaction coordinates and calculating the corresponding energies. The PES provides insights into the possible reaction pathways and their energetics.5. Investigate the excited state dynamics: To understand the time evolution of the excited state chemistry, perform non-adiabatic molecular dynamics simulations or quantum dynamics calculations. These methods can provide information about the relaxation pathways, lifetimes of the excited states, and the branching ratios of different photochemical reactions.By following these steps, one can optimize the excited state chemistry of benzene and gain a deeper understanding of its photochemical behavior. This information can be valuable for designing new photoactive materials, understanding atmospheric chemistry, and developing novel photonic devices.