Excited-state dynamics of molecules with classically driven trajectories and Gaussians

Abstract

Simulating the dynamics of a molecule initiated in an excited electronic state constitutes a rather challenging task for theoretical and computational chemistry, as such dynamics leads to a strong coupling between nuclear motion and electronic states, that is, a breakdown of the Born-Oppenheimer approximation. This New Views article proposes a brief overview on recent theoretical developments aiming at simulating the excited-state dynamics of molecules-nonadiabatic molecular dynamics-focusing in particular on strategies employing travelling basis functions to portray the dynamics of nuclear wavefunctions. We start by discussing the central equations for nonadiabatic molecular dynamics in a Born-Huang representation. We then propose a comparison between two commonly employed strategies to simulate the excited-state dynamics of molecular systems in their full configuration space, Ab Initio Multiple Spawning (AIMS) and Trajectory Surface Hopping (TSH). The equations of motion for the two techniques are compared and used to contrast their respective description of phenomena involving the decoherence of nuclear wavepackets. Some recent works and developments of the AIMS method are then summarised. This New Views article ends with a highlight on the Exact Factorisation of the molecular wavefunction and how this approach contrasts with the more conventional Born-Huang picture when it comes to the description of photophysical and photochemical processes.

Machine Learning for Electronically Excited States of Molecules

“… 145 , 415 − 417 They mostly rely on a combination of Gaussians to represent the nuclear wave function. 258 For example, the variational multiconfiguration Gaussian method (dd-vMCG) 418 offers a variational and thus accurate solution for the equations of motion. Also full multiple spawning 45 , 401 , 419 can be regarded as fully quantum mechanically by describing the wave function with a number of time-dependent Gaussian functions that follow classical trajectories with quantum mechanically determined time-dependent coefficients.…”

Modeling Spin-Crossover Dynamics

“…Here, we discuss some practical aspects of employing different electronic structure methods to model spin-crossover dynamics. For comprehensive review of electronic structure methods used in NAMD the reader is referred to (22,58,60,95). The success of NAMD simulations depend on the accuracy of the electronic structure methods used to obtain the energies, energy gradients, and couplings for multiple electronic states at different molecular geometries, as has been demonstrated in the recent TSH study of the spin-crossover dynamics in thioformaldehyde (96).…”

CHAPTER 2. Extraterrestrial Photochemistry: Principles and Applications

“…A system in an electronically excited state, however, will likely encounter regions on the excited-state PES with non-negligible coupling between nuclear and electronic degrees of freedom. 40 The adiabatic approximation is violated if two or more levels become so close that nuclear motion promotes an electronic transition from one state to another. A prototypical example of non-adiabatic behavior occurs near conical intersections, defined as crossings between potential energy surfaces of the same spin multiplicity.…”