Abstract
ABSTRACTSimulating 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.
Highlights
Many parts of our way to picture a molecule and its chemistry have been heavily influenced by the Born–Oppenheimer Approximation (BOA)
Applying the SPA0 and the independent first generation approximation (IFGA) to the full multiple spawning (FMS) equations of motion leads to the Ab Initio Multiple Spawning (AIMS) method,3 which allows for the description of the nonadiabatic dynamics of molecules in full dimensionality
These observations greatly motivated the development of approximate nonadiabatic dynamics strategies based on the Exact Factorisation [62,214,215], with methods like the coupled-trajectory mixed quantum/classical (CT-MQC) strategy having been applied to the excited-state dynamics of molecular systems [216–221]
Summary
Many parts of our way to picture a molecule and its chemistry have been heavily influenced by the Born–Oppenheimer Approximation (BOA). MCDTH still scales exponentially with the number of degrees of freedom, the base of the exponentiation is reduced to the number of single particle basis functions per degree of freedom (compared to the number of grid points in the standard method) Another strategy consists in expressing the nuclear wavefunctions using trajectory basis functions (TBFs). Note that apart from these strategies, a variety of other formalisms has been developed to describe nonBorn–Oppenheimer dynamics, such as semiclassical approaches [40–42], including e.g. mapping approaches [43–47], quantum-classical Liouville methods [48–50], Bohmian dynamics [51–54] or quantum trajectory mean-field dynamics [55] In this New Views article, we propose an introduction to the central equations of nonadiabatic molecular dynamics, highlighting the key required quantities for their solution – electronic structure and nuclear dynamics. We discuss new perspectives on excited-state dynamics offered by the so-called Exact Factorisation of the molecular wavefunction
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