Geometry optimization of a molecule refers to the process of finding the most stable minimum energy structure of a molecule by adjusting the positions of its atoms. This is done by minimizing the potential energy of the molecule with respect to the atomic coordinates. The optimized geometry represents the lowest energy conformation of the molecule in its ground electronic state.Electronic excitation energy is the energy required to promote an electron from its ground state to an excited state. The excitation energy is influenced by the spatial arrangement of atoms in the molecule, as it affects the distribution of electron density and the molecular orbitals.Geometry optimization can affect the electronic excitation energy in several ways:1. Changes in bond lengths and angles: Optimizing the geometry can lead to changes in bond lengths and angles, which in turn affect the overlap of atomic orbitals and the energy levels of molecular orbitals. This can cause changes in the energy gap between the ground and excited states, thus affecting the electronic excitation energy.2. Changes in conjugation and electronic delocalization: The optimized geometry may result in an increase or decrease in the extent of conjugation and electronic delocalization in the molecule. This can affect the energy levels of the molecular orbitals and the electronic excitation energy.3. Changes in the dipole moment: Geometry optimization can lead to changes in the molecular dipole moment, which can affect the interaction of the molecule with external electric fields and the transition dipole moment associated with electronic excitations. This can influence the electronic excitation energy and the probability of electronic transitions.4. Changes in the potential energy surface: The optimized geometry can affect the shape of the potential energy surface associated with the ground and excited states of the molecule. This can influence the vibrational frequencies and the coupling between electronic and nuclear motions, which can have an impact on the electronic excitation energy.In summary, geometry optimization of a molecule can significantly affect its electronic excitation energy by altering the molecular structure, electronic distribution, and potential energy surface. These changes can influence the energy levels of molecular orbitals, the energy gap between ground and excited states, and the probability of electronic transitions.