Assessment of Real-Time Time-Dependent Density Functional Theory (RT-TDDFT) in Radiation Chemistry: Ionized Water Dimer

Abstract

Ionization in the condensed phase and molecular clusters leads to a complicated chain of processes with coupled electron-nuclear dynamics. It is difficult to describe such dynamics with conventional nonadiabatic molecular dynamics schemes since the number of states swiftly increases as the molecular system grows. It is therefore attractive to use a direct electron and nuclear propagation such as the real-time time-dependent density functional theory (RT-TDDFT). Here we report a RT-TDDFT benchmark study on simulations of singly and doubly ionized states of a water monomer and dimer as a prototype for more complex processes in a condensed phase. We employed the RT-TDDFT based Ehrenfest molecular dynamics with a generalized gradient approximate (GGA) functional and compared it with wave-function-based surface hopping (SH) simulations. We found that the initial dynamics of a singly HOMO ionized water dimer is similar for both the RT-TDDFT/GGA and the SH simulations but leads to completely different reaction channels on a longer time scale. This failure is attributed to the self-interaction error in the GGA functionals and it can be avoided by using hybrid functionals with large fraction of exact exchange (represented here by the BHandHLYP functional). The simulations of doubly ionized states are reasonably described already at the GGA level. This suggests that the RT-TDDFT/GGA method could describe processes following the autoionization processes such as Auger emission, while its applicability to more complex processes such as intermolecular Coulombic decay remains limited.

Proton-transfer dynamics in ionized water chains using real-time time-dependent density functional theory

“…In this work we consistently employ the Ehrenfest dynamics approach but focus on a non-hybrid functional (PBE), as it turns out to be computationally feasible for a condensed system consisting of a large number of electronic states. We also confirm the results obtained by Chalabala et al [16] for the smallest system, that is, a dimer cation.…”

“…For instance, whether or not the transfer of a (photogenerated) hole to surface-water species leads to a fast separation of the proton remains an open question. Furthermore, the timescale of the proton-transfer reaction and whether it ought to be studied at the adiabatic or nonadiabatic level of theory [13][14][15][16][17][18] is also still an open question. These questions are difficult to address, largely due to the inherent complexity of the systems involved, in particular, due to large system sizes, the necessity for long simulation times, and potential contributions from nonadiabatic terms.…”

“…An extensive study focusing on the dynamics of ionized water monomers and dimers was carried by Chalabala et al [16]. They compared the results from two nonadiabatic approaches, namely, surface hopping and Ehrenfest dynamics.…”

“…As opposed to the averaged diabatic surface that the Ehrenfest method uses to evolve the electronic states, in surface hopping the time evolution of the system occurs on individual diabatic surfaces, with an intersurface transition determined by the instantaneous nonadiabatic couplings [57]. The authors of [16] found that both simulation methods predict a very similar statistics of trajectories, resulting in proton transfer when the Ehrenfest dynamics is carried out with a hybrid functional, and the reference surface hopping method employs a complete active space approach for computing the diabatic energy surfaces. In this work we consistently employ the Ehrenfest dynamics approach but focus on a non-hybrid functional (PBE), as it turns out to be computationally feasible for a condensed system consisting of a large number of electronic states.…”

“…At the end of the simulation period (t = 200 fs), a protontransferred structure is formed in only 13% of the initiated trajectories. On the other hand, using the BHLYP (hybrid) functional in Ehrenfest dynamics, Chalabala et al [16] found 95% of their trajectories resulting in proton-transfer events. We associate this difference to the fact that PBE spuriously determines the hemibonded structure to be lower in energy E PT→HB = −0.18 eV (see Table I).…”