The absorption spectra of a molecule are highly dependent on its geometrical configuration. When the structure of a molecule is altered, the energy levels of its molecular orbitals change, which in turn affects the energy differences between the ground and excited states. These energy differences determine the wavelengths of light that can be absorbed by the molecule, and thus the absorption spectra.Quantum chemical calculations can be used to predict and explain the changes in the excited states of a molecule when its structure is altered. These calculations involve solving the Schrödinger equation for the molecule, which provides information about the energy levels of the molecular orbitals and the corresponding electron configurations.When the geometrical configuration of a molecule changes, the spatial distribution of the atomic orbitals and their overlap also change. This affects the molecular orbitals formed through linear combinations of atomic orbitals LCAO . Consequently, the energy levels of the molecular orbitals change, leading to changes in the energy differences between the ground and excited states.To predict the changes in the absorption spectra, one can perform quantum chemical calculations for the different geometrical configurations of the molecule. This can be done using various computational chemistry software packages, such as Gaussian, ORCA, or GAMESS. These programs can calculate the molecular orbitals and their energies, as well as the excitation energies and oscillator strengths for the electronic transitions between the ground and excited states.By comparing the calculated excitation energies and oscillator strengths for the different geometrical configurations, one can predict how the absorption spectra will change upon altering the structure of the molecule. For example, if the energy difference between the ground and excited states decreases, the absorption peak will shift to longer wavelengths redshift , while an increase in the energy difference will result in a shift to shorter wavelengths blueshift .Additionally, changes in the geometrical configuration can also affect the intensity of the absorption peaks. If the oscillator strength of a particular electronic transition increases, the corresponding absorption peak will become more intense, and vice versa.In summary, quantum chemical calculations can be used to predict and explain the changes in the excited states and absorption spectra of a molecule when its geometrical configuration is altered. These calculations provide valuable insights into the relationship between molecular structure and spectroscopic properties, which can be useful for designing molecules with specific optical properties or for understanding the behavior of molecules in various environments.