Excited States in Quantum Chemistry: Insights from ZINDO, CIS, and TDDFT
Posted by Vivian Smith
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18 Aug 2023 05:59:06 am.
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ZINDO
What does ZINDO stand for? ZINDO is short for "Zerner's intermediate neglect of differential overlap." In the 1970s, Michael Zerner and his colleagues expanded on the original INDO method and developed ZINDO. It is a semi-empirical quantum chemistry method widely used in computational chemistry. ZINDO has two versions, one for calculating the ground state and the other for calculating the excited state. The version used for excited state calculations is called ZINDO/S or INDO/S, which computes the excited state using INDO/1 molecular orbitals to determine the electronic spectra. ZINDO allows researchers to perform essential tasks such as single-point energy calculations, determination of single-point forces, electronic excitations, geometry optimization, and transition-state optimization.
CIS
CIS, also known as configuration interaction with single excitations, is one of the most commonly used methods for obtaining excited state energies. It is also one of the simplest calculation methods to implement. Initially, scientists used Hartree-Fock theory to compute excited states. However, the electron-electron interactions in the average potential resulted in inaccuracies in the best energies obtained at the Hartree-Fock level, rendering the excited state calculations unsuccessful. By incorporating descriptions of correlated electron motions, CIS corrects the first and higher orders of the Hartree-Fock wavefunction. This correction overcomes the limitations of the Hartree-Fock method, enabling researchers to easily determine the energies of many excited states.
TDDFT
Time-dependent density-functional theory, abbreviated as TDDFT, is a quantum mechanical theory derived from density-functional theory (DFT). It finds wide application in various fields such as physics, (bio)chemistry, and materials science. TDDFT investigates the properties and dynamics of many-body systems in the presence of time-dependent potentials, such as electric and magnetic fields, with a formally exact and computationally efficient approach. The linear response function, which describes the changes in electron density resulting from variations in the external potential, exhibits a pole at the precise excitation energy of a system. Consequently, TDDFT has become a popular choice among scientists for calculating the energy of the excited state of an isolated system.
The calculation of excited states, as a frontier in quantum chemistry, plays a crucial role in accelerating the discovery and research of various things, including drugs, experimental materials, and industrial catalysts. It has garnered significant interest and pursuit from scientists and researchers across different fields. Efficient and accurate calculation of excited state energies is the primary concern for these individuals. The three methods mentioned above precisely address their needs. With the application of these methods, along with advanced technologies and extensive knowledge, the calculation of excited states will continue to become more accessible and efficient in the future.
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