Abstract
The main shortcoming of time-dependent density functional theory (TDDFT) regarding its use for nonadiabatic molecular dynamics (NAMD) is its incapability to describe conical intersections involving the ground state. To overcome this problem, we combine Fermi smearing (FS) DFT with a fractional-occupation variant of the Tamm-Dancoff approximation (TDA) of TDDFT in the generalized gradient approximation. The resulting method (which we denote as FS-TDA) gives access to ground- and excited-state energies, gradients, and nonadiabatic coupling vectors, which are physically correct even in the vicinity of S1-S0 conical intersections. This is shown for azobenzene, a widely used photoswitch, via single point calculations and NAMD simulations of its cis-trans photoisomerization. We conclude that FS-TDA may be used as an efficient alternative to investigate these processes.
Highlights
Time-dependent density functional theory (TDDFT)1–3 can be regarded as the workhorse of today’s theoretical photochemistry.4Being computationally less demanding than the other methods, it gives access to excitation spectra,5,6 excited-state properties,7,8 and even nonadiabatic molecular dynamics (NAMD)9–14 of good quality even for large molecular systems
Excited-state energies and properties are obtained from a subsequent Tamm–Dancoff approximation (TDA)-TDDFT calculation, which accounts for the fractional occupation numbers of the ground-state configuration without requiring derivatives with respect to n
We have introduced the Fermi smearing (FS)-TDA ansatz to perform excitedstate calculations and dynamics at the TDDFT level using fractional occupation numbers
Summary
Time-dependent density functional theory (TDDFT) can be regarded as the workhorse of today’s theoretical photochemistry.. More demanding approaches are spin-flip TDDFT, combining DFT with a multireference method, or calculating the electronic structure as an ensemble of densities or states.. More demanding approaches are spin-flip TDDFT, combining DFT with a multireference method, or calculating the electronic structure as an ensemble of densities or states.34,35 Practical approaches of the latter are thermal DFT, restricted-ensemble Kohn–Sham (KS) DFT, or thermally assisted-occupation (TAO) DFT.. In contrast to previous works based on DFT (e.g., TAO-DFT42–45), we solely use n to converge our ground-state calculations applying a constant T, while the exchange-correlation functional does not explicitly depend on the occupation numbers n. Excited-state energies and properties are obtained from a subsequent TDA-TDDFT calculation, which accounts for the fractional occupation numbers of the ground-state configuration without requiring derivatives with respect to n.
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