*Ab Initio* Nonadiabatic Molecular
Dynamics with Hole–Hole Tamm–Dancoff Approximated Density
Functional Theory

journal contribution

posted on 20.08.2020 by Jimmy
K. Yu, Christoph Bannwarth, Edward G. Hohenstein, Todd J. Martínez#### journal contribution

Any type of content formally published in an academic journal, usually following a peer-review process.

The study of photoinduced dynamics
in chemical systems necessitates
accurate and computationally efficient electronic structure methods,
especially as the systems of interest grow larger. The linear response
hole–hole Tamm–Dancoff approximated (

*hh*-TDA) density functional theory method was recently proposed to satisfy such demands. The*N*-electron electronic states are obtained by means of double annihilations on a doubly anionic (*N*+ 2)-electron reference state, allowing for the ground and excited states to be formed on the same footing and thus enabling the correct description of conical intersections. Dynamic electron correlation effects are incorporated by means of the exchange–correlation functional. The accuracy afforded by the simultaneous treatment of static and dynamic correlation in addition to the relatively low computational cost, comparable to that of time-dependent density functional theory (TDDFT), makes it a promising*ab initio*electronic structure method for on-the-fly generation of potential energy surfaces in nonadiabatic dynamics simulations of photochemical systems, particularly those for which the nπ* and ππ* electronic excitations are most relevant. Here, we apply the*hh*-TDA method to nonadiabatic dynamics simulations of prototypical photochemical processes. First, we demonstrate the ability of*hh*-TDA to adequately describe conical intersection geometries. We next examine its ability to describe the ultrafast excited state dynamics of photoexcited ethylene through an*ab initio*multiple spawning (AIMS) dynamics simulation. Finally, we present an alternative variant of the*hh*-TDA method, which uses orbitals from a fractional occupation number Kohn–Sham (FON-KS) calculation applied to an ensemble with*N*-electrons. The resulting method is termed floating occupation molecular orbital*hh*-TDA (FOMO-*hh*-TDA). This scheme allows us to combine*hh*-TDA with global hybrid functionals and allows us to avoid unbound valence orbitals that may result from an (*N*+ 2)-electron self-consistent field (SCF) procedure. FOMO-*hh*-TDA–BHLYP faithfully reproduces the nonadiabatic dynamics of*trans*-azobenzene (TAB) and is used to characterize the excited state decay pathways from the first (nπ*) excited state.