The theoretical calculation of the absorption spectrum of a molecule involves determining the energies at which a molecule absorbs light, leading to electronic transitions between different energy levels. This can be achieved using quantum chemistry methods, such as time-dependent density functional theory TD-DFT or configuration interaction singles CIS calculations. These methods provide the excitation energies and oscillator strengths for the electronic transitions, which can be used to simulate the absorption spectrum.The specific electronic transitions responsible for each peak in the absorption spectrum are characterized by the promotion of an electron from an occupied molecular orbital MO to an unoccupied MO. These transitions can be classified into different types based on the nature of the involved orbitals:1. -* transitions: These transitions occur when an electron is promoted from a bonding orbital to a * antibonding orbital. They are common in molecules with conjugated systems, such as aromatic compounds and polyenes. These transitions usually result in strong absorption bands in the ultraviolet UV region.2. n-* transitions: In these transitions, an electron is promoted from a non-bonding n orbital, typically associated with lone pairs, to a * antibonding orbital. These transitions are common in molecules with carbonyl groups, nitro groups, or other functional groups with lone pairs. They usually result in weaker absorption bands compared to -* transitions and are found in the UV or visible region.3. -* transitions: These transitions involve the promotion of an electron from a bonding orbital to a * antibonding orbital. They are common in molecules with single bonds, such as alkanes and alcohols. These transitions typically require higher energies and result in absorption bands in the far-UV region.4. n-* transitions: In these transitions, an electron is promoted from a non-bonding n orbital to a * antibonding orbital. They are less common and usually result in weak absorption bands in the UV region.5. Charge-transfer transitions: These transitions involve the transfer of an electron from a donor orbital usually a non-bonding or orbital to an acceptor orbital usually a * or * orbital within the molecule or between different parts of the molecule. These transitions can result in strong absorption bands in the visible or near-UV region and are often responsible for the color of many organic compounds.By analyzing the calculated excitation energies and the involved molecular orbitals, one can assign the specific electronic transitions responsible for each peak in the absorption spectrum.