Preprint: Details in the surface hopping algorithm

Having discussed the influence of electronic structure methods in surface hopping dynamics in the last post and paper, we can now proceed to the surface hopping algorithm itself. To our surprise, algorithmic details such as the decoherence correction (energy-based decoherence or augmented FSSH), momentum rescaling and the treatment of frustrated hops can make a big difference. This is what we investigated in our new preprint “Strong Influence of Decoherence Corrections and Momentum Rescaling in Surface Hopping Dynamics of Transition Metal Complexes” available on ChemArxiv.

To have a well-defined reference, we used our new implementation of vibronic coupling models for surface hopping, which allows us to have a one-to-one comparison with accurate quantum dynamics computed at the MCTDH level of theory. As model system, we used a rhenium complex and studied its ultrafast intersystem crossing dynamics from the singlet to the triplet manifold following previous studies by our collaborators in Strasbourg [JCTC (2017), PCCP (2018)].

Paper: Electronic structure methods for dynamics simulations

The challenge about running photodynamics simulations is that the computational cost is often so high that one might have to compromise in terms of the electronic structure method used. One is tempted to just check the vertical excitations at one geometry and run the dynamics if those look alright. How this can go wrong is investigated in the paper “The Influence of the Electronic Structure Method on Intersystem Crossing Dynamics. The Case of Thioformaldehyde” that just appeared in JCTC. Take a look if you are interested.

 

Copyright 2019 American Chemical Society.

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Paper: Delayed fluorescence

Small modifications can make a big difference. Swapping anthraquinone for a thiophene based acceptor in a donor-acceptor-donor system produces the desired red shift in the emission but limits its quantum efficiency. This conclusion was drawn from a recent paper lead by Stephanie Montanaro and Iain Wright at Loughborough: Red-shifted delayed fluorescence at the expense of photoluminescence quantum efficiency – an intramolecular charge-transfer molecule based on a benzodithiophene-4,8-dione acceptor which just a appeared in PCCP.

Computations reveal that the reason for the reduced emission quantum efficiency lies in the presence of low-lying locally excited states (4 triplets and one singlet) on the central unit.

Release of TheoDORE 2.0 (beta)

Version 2.0 of the TheoDORE wavefunction analysis package has been released, download below. The two main features of TheoDORE 2.0 are the computation of conditional electron densities and compatibility with python3.

Conditional electron densities can be used for the visualisation of excited-state electron correlation, see ChemPhotoChem (2019). Below, the application of this method to a PPV oligomer is shown. Here, the probe hole (red) is always fixed on the terminal phenyl ring and the different shapes for the conditional electron density (blue) for the first six excited states is observed. One can see that for the different states the electron is either repelled, attracted or unaffected by the hole.

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Talks: Marseille and Montpellier

This week Felix will give a seminar talks at Marseille and Montpellier entitled”New Tools for Computational Photochemistry: Wavefunction Analysis and Dynamics.” The talk briefly summarises a number of computational methods developed.

Wavefunction analysis methods:

Methods for nonadiabatic dynamics simulations:

You can download the slides here:

Talk: Ionic and covalent states

Tomorrow, Felix will give a talk at the Computational Molecular Science Meeting in Warwick: “Understanding ionic and covalent wavefunction character without valence bond theory”.

The talk presents some new ideas on a long-standing question in computational chemistry, the connection between valence-bond theory and molecular orbital theory. In particular, the talk will be explore how two concepts from valence-bond theory, ionic and covalent wavefunction character, can be reconstructed from general quantum chemistry computations performed in the molecular orbital picture. The talk is based on the following paper in ChemPhotoChem.

You can download the slides here:

Paper: Visualisation of electron correlation

Felix’s first purely Loughborough grown paper just appeared as a Communication in ChemPhotoChem: “Visualisation of Electronic Excited‐State Correlation in Real Space“. The paper explores a new method for visualising electron correlation exemplified in the case excited states represented in the electron/hole picture. The idea is to fix the hole on one fragment of the system and to observe how the excited electron adjusts to this position.

Below, I am showing this analysis for the lowest 10 excited states of the system. The hole (shown in red) is always located on the second thiophene unit from the right. The electron (shown in blue) adjusts in different ways to this hole position, either moving toward it or going away from it.

As shown above, the method offers a new and intuitive way of viewing correlated exciton wavefunctions. It was also shown in this paper how the method allows to naturally distinguish between ionic and covalent states of naphthalene without any explicit reference to valence bond theory.

Talk: New Tools for Computational Photochemistry

Tomorrow, Felix will give a seminar talk at the University of Nottingham – “New Tools for Computational Photochemistry: Wavefunction Analysis and Dynamics.” The talk briefly summarises a number of computational methods developed.

Wavefunction analysis methods:

Methods for nonadiabatic dynamics simulations:

You can download the slides here: