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Updated: Jun 24, 2025
08:04
Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids
Published on: May 27, 2020
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Excited State-Specific CASSCF Theory for the Torsion of Ethylene
Sandra Saade1,2, Hugh G A Burton1
1Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
Journal of Chemical Theory and Computation
|June 7, 2024
Summary
State-specific complete active space self-consistent field (SS-CASSCF) theory shows promise for excited state calculations. However, unphysical discontinuities hinder its practical application in computational photochemistry.
Area of Science:
- Quantum Chemistry
- Computational Photochemistry
- Electronic Structure Theory
Background:
- State-specific complete active space self-consistent field (SS-CASSCF) theory is a developing method for predicting excited energy surfaces.
- Its accuracy and feasibility for photochemical systems require further evaluation.
Purpose of the Study:
- To assess the performance of SS-CASSCF theory for ethylene's double bond rotation.
- To compare SS-CASSCF with state-averaged methods and evaluate orbital optimization benefits.
Main Methods:
- Investigated low-lying ground and excited states of ethylene using SS-CASSCF.
- Employed a minimal (2e,2o) active space for state-specific calculations.
- Compared results with state-averaged calculations using larger active spaces.
Main Results:
- SS-CASSCF with a minimal active space achieved accuracy comparable to state-averaged methods with larger spaces.
- Optimizing orbitals for each excited state enhanced wave function spatial diffusivity.
- Incorrect ordering of state-specific solutions led to unphysical discontinuities in the potential energy surface.
Conclusions:
- SS-CASSCF offers a potentially accurate and efficient approach for excited state calculations.
- Theoretical challenges, particularly state ordering, must be resolved for practical computational photochemistry applications.
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