15 research outputs found
Troubleshooting Time-Dependent Density-Functional Theory for Photochemical Applications: Oxirane
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.Comment: 19 pages, 17 figures, submitted to the Journal of Chemical Physic
Suitable coordinates for quantum dynamics: Applications using the multiconfiguration time-dependent Hartree (MCTDH) algorithm
International audienc
Different Flavors of Nonadiabatic Molecular Dynamics
The BornâOppenheimer approximation constitutes a cornerstone of our understanding of molecules and their reactivity, partly because it introduces a somewhat simplified representation of the molecular wavefunction. However, when a molecule absorbs light containing enough energy to trigger an electronic transition, the simplistic nature of the molecular wavefunction offered by the BornâOppenheimer approximation breaks down as a result of the now nonânegligible coupling between nuclear and electronic motion, often coined nonadiabatic couplings. Hence, the description of nonadiabatic processes implies a change in our representation of the molecular wavefunction, leading eventually to the design of new theoretical tools to describe the fate of an electronicallyâexcited molecule. This Overview focuses on this quantityâthe total molecular wavefunctionâand the different approaches proposed to describe theoretically this complicated object in nonâBornâOppenheimer conditions, namely the BornâHuang and ExactâFactorization representations. The way each representation depicts the appearance of nonadiabatic effects is then revealed by using a model of a coupled protonâelectron transfer reaction. Applying approximations to the formally exact equations of motion obtained within each representation leads to the derivation, or proposition, of different strategies to simulate the nonadiabatic dynamics of molecules. Approaches like quantum dynamics with fixed and timeâdependent grids, traveling basis functions, or mixed quantum/classical like surface hopping, Ehrenfest dynamics, or coupledâtrajectory schemes are described in this Overview