Quantum dynamics in macrosystems with several coupled electronic states: hierarchy of effective Hamiltonians

TL;DR

This paper develops a hierarchical approach to simulate nonadiabatic quantum dynamics in macrosystems with multiple electronic states, enabling exact numerical calculations by reducing environmental complexity through effective Hamiltonians.

Contribution

It introduces a hierarchy of effective Hamiltonians for environmental modes, allowing precise quantum dynamics simulations of complex macrosystems with multiple coupled electronic states.

Findings

Exact quantum dynamics can be computed on a given time-scale.

The environment's influence is captured through sequentially coupled effective modes.

The method allows evaluation of observables like spectra and electronic populations.

Abstract

We address the nonadiabatic quantum dynamics of macrosystems with several coupled electronic states, taking into account the possibility of multi-state conical intersections. The general situation of an arbitrary number of states and arbitrary number of nuclear degrees of freedom (modes) is considered. The macrosystem is decomposed into a system part carrying a few, strongly coupled modes, and an environment, comprising the vast number of remaining modes. By successively transforming the modes of the environment, a hierarchy of effective Hamiltonians for the environment is constructed. Each effective Hamiltonian depends on a reduced number of effective modes, which carry cumulative effects. By considering the system's Hamiltonian along with a few members of the hierarchy, it is shown mathematically by a moment analysis that the quantum dynamics of the entire macrosystem can be numerically exactly computed on a given time-scale. The time scale wanted defines the number of effective Hamiltonians to be included. The contribution of the environment to the quantum dynamics of the macrosystem translates into a sequential coupling of effective modes. The wavefunction of the macrosystem is known in the full space of modes, allowing for the evaluation of observables such as the time-dependent individual excitation along modes of interest, as well a spectra and electronic-population dynamics.