Abstract
We report an efficient analytical implementation of first-order nonadiabatic derivative couplings between arbitrary Born-Oppenheimer states in the hybrid time-dependent density functional theory (TDDFT) framework using atom-centered basis functions. Our scheme is based on quadratic response theory and includes orbital relaxation terms neglected in previous approaches. Simultaneous computation of multiple derivative couplings and energy gradients enables efficient multistate nonadiabatic molecular dynamics simulations in conjunction with Tully's fewest switches surface hopping (SH) method. We benchmark the thus obtained multistate TDDFT-SH scheme by simulating ultrafast decay of UV-photoexcited thymine, for which accurate gas-phase data from ultrafast spectroscopy experiments are available. The calculations predict a fast 153 fs decay from the bright S2 to the dark S1 excited state, followed by a much slower 14 ps S1 deactivation to the ground state; statistical uncertainties were estimated using bootstrap sampling. These results agree well with the experimentally observed time constants of 100-200 fs and 5-7 ps, respectively, unlike previous multiconfigurational self-consistent field and second-order algebraic diagrammatic construction calculations. Furthermore, our results support the S1-trapping hypothesis [J. J. Szymczak et al., J. Phys. Chem. A, 2009, 113, 12686-12693]. For thymine, the computational cost of a single TDDFT-SH time-step including the lowest 3 states, all couplings and gradients, is ∼5 times larger than the cost of a single Born-Oppenheimer dynamics time step for the ground state in our implementation. Thus, ps nonadiabatic dynamics simulations using multistate hybrid TDDFT-SH for systems with up to ∼100 atoms are possible without drastic approximations on single workstation nodes. Our implementation will be made available through Turbomole.
Highlights
We report an efficient analytical implementation of state-to-state nonadiabatic couplings and multistate time-dependent density functional theory (TDDFT)-surface hopping (SH) dynamics, which builds on these theoretical results and the implementation of the TDDFT quadratic response properties reported by our group previously.[36]
Three decay channels of UV-photoexcited thymine have been distinguished on the basis of gas phase femtosecond pump–probe transient ionization spectroscopy,[37,38,40,41,42,43] femtosecond time-resolved photoelectron spectroscopy (TRPES),[39] and ultrafast X-ray Auger,[44] see Table 1: (i) a prompt signal with a time constant near 100–200 fs (ii) a fast signal with time constant of 5.1–7 ps and (iii) a slow decay conventionally attributed to intersystem crossing that is longer than 100 ps
The predicted S2 lifetime of 17 fs and S1 lifetime of 420 fs lead to the interpretation that the ultrashort signal corresponds to internal conversion S2 - S1, the prompt signal corresponds to internal conversion to the ground state (S1 - S0) and the fast component corresponds to thermalization on the ground state.[48]
Summary
Nonadiabatic molecular dynamics[1] (NAMD) using a combination of time-dependent density functional theory[2,3,4,5,6,7] (TDDFT) and surface hopping[8] (SH) has emerged as a versatile tool for studying and analyzing complex photochemical transformations.[9,10,11] TDDFT-SH has proven capable of predicting the kinetics of pericyclic ringopening and -closing reactions,[12,13,14,15,16,17,18,19,20] the reactivity of photoexcited metal oxides[21] such as perovskites[22] and titania,[23,24,25] and the branching ratios, kinetic energy distributions, and mass distributions of photodissociation reactions.[26,27]. Its decay has been extensively studied experimentally[37,38,39,40,41,42,43,44] and computationally.[43,45,46,47,48,49,50,51,52,53] the deactivation mechanism of UV-photoexcited thymine remains controversial.[37,45,53] In particular, prior NAMD-SH simulations using multiconfiguration selfconsistent field (MCSCF) and the second-order algebraic diagrammatic construction [ADC(2)] methods lead to excited-state lifetimes hard to reconcile with each other and with ultrafast pump–probe experiments.[37,43,50,52].
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