Do we need delocalised wavefunctions for the excited state dynamics of 1,1-difluoroethylene?

TL;DR

This study investigates whether using localized or delocalized wavefunctions affects the accuracy of excited state quantum dynamics simulations in 1,1-difluoroethylene, finding minimal impact on key observables.

Contribution

The paper introduces a model Hamiltonian for excited state dynamics that accounts for symmetry and compares localized versus delocalized wavefunctions, showing their negligible effect on results.

Findings

Different initial wavefunction localizations do not significantly alter excited state populations.

The model Hamiltonian accurately reproduces experimental absorption spectra.

Delocalization assumptions may be unnecessary for similar small symmetric molecules.

Abstract

In this work we set up a model Hamiltonian to study the excited state quantum dynamics of 1,1-difluoroethylene, a molecule that has equivalent atoms exchanged by a torsional symmetry operation leading to equivalent minima on the potential energy surface. In systems with many degrees of freedom where the minimum energy geometry is not unique, the ground state wavefunction will be delocalised among multiple minima. In this small test system, we probe the excited state dynamics considering localised (in a single minimum) and delocalised (spread over among multiple minima) wavefunctions and check whether this choice would influence the final outcome of the quantum dynamics calculations. Our molecular Hamiltonian comprises seven electronic states, including valence and Rydberg states, computed with the MS-CASPT2 method and projected onto the vibrational coordinates of the twelve normal modes of 1,1-difluoroethylene in its vibrational ground state. This Hamiltonian has been symmetrised along the torsional degree of freedom to make both minima completely equivalent and the model is supported by the excellent agreement with the experimental absorption spectrum. Quantum dynamics results show that the different initial conditions studied do not appreciably affect the excited state populations or the absorption spectrum when the dynamics is simulated assuming a delta pulse excitation.