Excitation Energy Calculations in Quantum Chemistry

Excitation energy calculations lie at the heart of understanding how molecules absorb and emit light, govern photochemical reactivity and underpin the design of functional materials such as organic light-emitting diodes, solar cells and phototherapeutic agents. In quantum chemistry, these energies are obtained by solving the electronic Schrödinger equation for both the ground state and a manifold of excited states, either by wavefunction-based approaches or by density functional approximations. Wavefunction methods include variants of coupled cluster theory and multireference formalisms that systematically incorporate electron correlation, while density functional approaches rely on extensions such as time-dependent density functional theory to capture excitation phenomena at reduced cost. A reliable prediction of vertical excitation energies, potential energy surfaces and singlet–triplet gaps requires careful consideration of basis-set completeness, orbital relaxation and vibronic coupling. Benchmark studies and the emergence of local and embedding techniques are extending the reach of high-accuracy methods towards complex systems containing hundreds of atoms. This body of work directly informs the rational design of molecular chromophores, photoactive biomolecules and advanced optoelectronic devices by delivering quantitative insights into their electronic spectra and excited-state dynamics.

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Excitation Energy Calculations in Quantum Chemistry publication trend

The graph below shows the total number of articles in excitation energy calculations in quantum chemistry across all publications each year (not limited to Nature Index journals).

Technical terms

Excitation energy: The energy difference between an electronic ground state and an excited state of a molecule.

Time-dependent density functional theory (TDDFT): An extension of density functional theory that describes excited states by treating the electron density as a time-dependent variable.

Unrestricted density functional theory (UDFT): A form of DFT in which alpha and beta spin electrons occupy different spatial orbitals, allowing spin polarisation in open-shell systems.

Coupled cluster methods: Wavefunction-based approaches that model electron correlation by an exponential cluster operator acting on a reference determinant, often including singles, doubles and perturbative triples.

Charge-transfer state: An excited state in which an electron is relocated from one region or fragment of a molecule to another, leading to spatial separation of charge.

References

1. Benchmark Study on Phosphorescence Energies of Anthraquinone Compounds: Comparison between TDDFT and UDFT. Molecules (2023).
2. Evaluating the interactions between vibrational modes and electronic transitions using frontier orbital energy derivatives. Chemical Communications (2024).
3. A New Benchmark Set for Excitation Energy of Charge Transfer States: Systematic Investigation of Coupled Cluster Type Methods. Journal of Chemical Theory and Computation (2020).