Time-dependent variational dynamics for nonadiabatically coupled nuclear and electronic quantum wavepackets in molecules

Abstract

We propose a methodology to unify electronic and nuclear quantum wavepacket dynamics in molecular processes including nonadiabatic chemical reactions. The canonical and traditional approach in the full quantum treatment both for electrons and nuclei rests on the Born–Oppenheimer fixed nuclei strategy, the total wavefunction of which is described in terms of the Born–Huang expansion. This approach is already realized numerically but only for small molecules with several number of coupled electronic states for extremely hard technical reasons. Besides, the stationary-state view of the relevant electronic states based on the Born–Oppenheimer approximation is not always realistic in tracking real-time electron dynamics in attosecond scale. We therefore incorporate nuclear wavepacket dynamics into the scheme of nonadiabatic electron wavepacket theory, which we have been studying for a long time. In this scheme thus far, electron wavepackets are quantum mechanically propagated in time along nuclear paths that can naturally bifurcate due to nonadiabatic interactions. The nuclear paths are in turn generated simultaneously by the so-called matrix force given by the electronic states involved, the off-diagonal elements of which represent the force arising from nonadiabatic interactions. Here we advance so that the nuclear wavepackets are directly taken into account in place of path (trajectory) approximation. The nuclear wavefunctions are represented in terms of the Cartesian Gaussians multiplied by plane waves, which allows for feasible calculations of atomic and molecular integrals together with the electronic counterparts in a unified manner. The Schrödinger dynamics of the simultaneous electronic and nuclear wavepackets are to be integrated by means of the dual least action principle of quantum mechanics [K. Takatsuka, J. Phys. Commun. 4, 035007 (2020)], which is a time-dependent variational principle. Great contributions of Vincent McKoy in the electron dynamics in the fixed nuclei approximation and development in time-resolved photoelectron spectroscopy are briefly outlined as a guide to the present work.

Molecular collisions, photoionization and dynamics: honouring Professor Vincent McKoy

“…New experimental data on the fragmentation of the tyrosine amino acid molecule and the formation of ionised products due to the lowenergy electron impact were presented by Tamuliene et al [5]. Takatsuka [6] has presented a methodology to unify electronic and nuclear quantum wavepacket dynamics in molecular processes including nonadiabatic chemical reactions whereas a R-matrix theory implemented using the pseudostate formalism has been used to calculate elastic and excitation processes in electron interactions with N 2 H and HCO [7]. Suarez-Moreno et al [8] have reported studies on electron interactions with formamide clusters as a model system to understand phenomena relevant to astrophysical, prebiotic and radiobiological processes.…”

Suppression of Charge Recombination by Auxiliary Atoms in Photoinduced Charge Separation Dynamics with Mn Oxides: A Theoretical Study

“…Most of theories of nonadiabatic transitions, and those by the Truhlar group as well, rest on the so-called Born–Huang representation [ 37 ], in which time-dependent dynamics of molecules are considered exclusively in the nuclear wave functions, which are supposed to run on the so-called static potential energy hyper-surface (PES) created by time-independent electronic wave functions. Our approach in this paper is, in contrast, based on the time-dependent electronic wave functions, and the nuclear kinematic interactions are taken into account through the coupling of electronic wave-packets [ 38 , 39 , 40 , 41 , 42 ]. In this representation, electron wave-packets induced by photoexcitation are directly tracked in a real-time scale.…”