Troubleshooting Time-Dependent Density-Functional Theory for Photochemical Applications: Oxirane

TL;DR

This paper evaluates the effectiveness of TDDFT with TDA in modeling oxirane photochemistry, demonstrating its practical utility and addressing common computational challenges in excited-state calculations.

Contribution

It shows that TDA is essential for stable and accurate TDDFT calculations of oxirane's photochemical reactions, validating its use in this context.

Findings

TDA prevents triplet and singlet instabilities in TDDFT calculations.

TDDFT/TDA provides qualitatively correct excited-state potential energy surfaces.

Practical challenges in modeling oxirane do not hinder TDDFT's qualitative accuracy.

Abstract

The development of analytic-gradient methodology for excited states within conventional time-dependent density-functional theory (TDDFT) would seem to offer a relatively inexpensive alternative to better established quantum-chemical approaches for the modeling of photochemical reactions. However, even though TDDFT is formally exact, practical calculations involve the use of approximate functionals, in particular the TDDFT adiabatic approximation, whose use in photochemical applications must be further validated. Here, we investigate the prototypical case of the symmetric CC ring opening of oxirane. We demonstrate by direct comparison with the results of high-quality quantum Monte Carlo calculations that, far from being an approximation on TDDFT, the Tamm-Dancoff approximation (TDA) is a practical necessity for avoiding triplet instabilities and singlet near instabilities, thus helping maintain energetically reasonable excited-state potential energy surfaces during bond breaking. Other difficulties one would encounter in modeling oxirane photodynamics are pointed out but none of these is likely to prevent a qualitatively correct TDDFT/TDA description of photochemistry in this prototypical molecule.