Excited state dynamics of a molecule refer to the behavior of a molecule when one or more of its electrons are promoted to higher energy levels, leading to an electronically excited state. These dynamics are influenced by changes in the electronic structure of the molecule, which can result from factors such as alterations in molecular geometry, chemical bonding, or external perturbations like light absorption. Understanding these excited state dynamics is crucial for various applications, including photochemistry, photophysics, and spectroscopy.The variation in excited state dynamics with changes in electronic structure can be understood through several factors:1. Potential energy surfaces PES : When the electronic structure of a molecule changes, its potential energy surfaces, which describe the energy of the molecule as a function of its nuclear coordinates, are also altered. This can lead to different excited state dynamics, such as changes in the relaxation pathways, reaction rates, and product distributions.2. Conical intersections: Changes in electronic structure can also affect the locations and properties of conical intersections, which are points where two or more potential energy surfaces intersect. Conical intersections play a crucial role in non-adiabatic processes, where the molecule can switch between different electronic states. The presence and properties of these intersections can significantly influence the excited state dynamics.3. Coupling between electronic states: The coupling between different electronic states can be affected by changes in the electronic structure. This coupling can lead to energy transfer between states, affecting the overall excited state dynamics and relaxation pathways.To obtain information about excited state dynamics and the influence of electronic structure changes, ab initio calculations in computational chemistry can be employed. Ab initio methods are based on the principles of quantum mechanics and do not rely on empirical data. Some common ab initio methods used to study excited state dynamics include:1. Time-dependent density functional theory TD-DFT : TD-DFT is an extension of density functional theory DFT that allows for the calculation of excited state properties. It can provide information about the electronic structure and excited state dynamics of molecules, including vertical excitations, oscillator strengths, and transition moments.2. Configuration interaction CI and multi-configurational self-consistent field MCSCF methods: These methods account for the correlation between electrons and can provide accurate descriptions of excited state dynamics. They can be used to calculate potential energy surfaces, transition moments, and other properties relevant to excited state processes.3. Time-dependent wavefunction methods: These methods, such as time-dependent Hartree-Fock TD-HF and time-dependent coupled cluster TD-CC , can provide accurate descriptions of excited state dynamics by solving the time-dependent Schrödinger equation. They can provide information about the evolution of the electronic structure and the coupling between different electronic states.By employing these ab initio methods, researchers can gain insights into the excited state dynamics of molecules and understand how changes in electronic structure affect these dynamics. This information can be used to design new molecules with desired properties, optimize chemical reactions, and develop novel materials for various applications.