Troubleshooting time-dependent density-functional theory for photochemical applications: oxirane

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• Time-dependent density-functional theory (TDDFT) offers a cost-effective method for modeling photochemical reactions.
• Practical TDDFT calculations rely on approximate functionals, necessitating validation, particularly the adiabatic approximation.

Purpose of the Study:

• To investigate the role of the Tamm-Dancoff approximation (TDA) in TDDFT for modeling photochemical reactions.
• To validate the use of TDA against high-quality quantum Monte Carlo (QMC) calculations for the oxirane ring-opening reaction.

Main Methods:

• Analytic-gradient methodology for excited states within TDDFT.
• Comparison of TDDFT results with and without the Tamm-Dancoff approximation against Quantum Monte Carlo calculations.
• Investigation of the symmetric CC ring opening of oxirane as a model system.

Main Results:

• The Tamm-Dancoff approximation is crucial for avoiding triplet and singlet instabilities in excited-state potential energy surfaces.
• TDA is a practical necessity, not an approximation, for maintaining energetically reasonable excited-state surfaces during bond breaking.
• Direct comparison with QMC confirmed the importance of TDA in TDDFT calculations for photodynamics.

Conclusions:

• The Tamm-Dancoff approximation is vital for the reliable application of TDDFT to photochemical reaction modeling.
• Addressing potential instabilities is key for accurate excited-state potential energy surface calculations.
• Further challenges in modeling oxirane photodynamics are identified.