Excited states of diradicals

Computations on diradicals are not only difficult in terms of choosing an appropriate electronic structure method but in many cases it is also quite challenging to make sense of the results obtained. To tackle this problem we developed a detailed characterisation scheme for the excited states of diradicals in our new paper Classification and quantitative characterisation of the excited states of π-conjugated diradicals that just appear in Faraday Discussions.

Our paper builds on earlier work by Salem et al. and Stuyver et al. in terms of formally characterising the states as diradical and zwitterionic within a two-orbital two-electron model (TOTEM). On top of this, we have provided practical protocols for recognising these states in realistic computations. Using our tools in the case of the para-quinodimethane molecule as a model system, we show that it is possible to identify the states arising from the TOTEM but also that the π-conjugated bridge plays a crucial role producing more complicated states than one would have expected from the simple TOTEM.

Matrix-free hyperfluorescence

Hyperfluorescence is an emerging technique for generating highly efficient OLEDs by combining a triplet harvester with a bright emitter molecule. Current devices are overly complex due to the number of components involved hampering practical application. A new paper, led by Hugo Bronstein from the University of Cambridge presents an important step toward solving this problem. The idea is to encapsulate the emitter, thus, avoiding the need for a high-gap matrix. The approach is presented in the paper Suppression of Dexter transfer by covalent encapsulation for efficient matrix-free narrowband deep blue hyperfluorescent OLEDs, which just appeared in Nature Materials.

Post-Doc Position on the Dynamics of High-Energy Materials

Applications are invited for a postdoctoral research associate position in computational chemistry at Loughborough University. This is a DSTL funded position to work with Dr Kenny Jolley and Dr Felix Plasser (Chemistry, Loughborough University) on predicting the crystal structure, dynamics and physical properties of energetic materials.

The successful applicant will conduct molecular dynamics (MD) and accelerated MD simulations on energetic compounds. Bulk physical properties, stability to shock and heat, and reaction pathways will be modelled and compared to experimental data. In a later stage, we will perform time-resolved simulations of explosion processes.

This position is ideally suited for an ambitious early career researcher with a background in computational chemistry and materials modelling. The successful candidate will be highly motivated with a strong research track record and a desire to pursue multidisciplinary research.

Feel free to contact me or Dr Kenny Jolley for informal equiries.

Closing date for applications is 24/11/2023, please follow this link for further info.

Ionic states

Our new paper Quantification of the Ionic Character of Multiconfigurational Wave Functions: The Qat Diagnostic just appeared in the Journal of Physical Chemistry A.

The paper deals with the fact that the widely used CASSCF method, if not used carefully, can yield large errors (1-2 eV) in vertical excitation energies. This problem arises for ionic states, as defined within valence bond theory. Within this work we developed a simple diagnostic to identify ionic states. We found a good correlation between the new diagnostic (Qta) and the error, as shown in the figure above.

We hope that the new diagnostic will be useful similar to analogous diagnostics identifying charge transfer states in TDDFT computations. This will give users the possibility to spot potential problems quickly.

On-going work is concerned with going from just diagnosing the problem to developing a numerical correction term to fix the problem.

Release of TheoDORE 3.1

Version 3.1 of the TheoDORE wavefunction analysis package is available. Download the current version below.

New features of TheoDORE 3.1

  • Transition charges for ionic/covalent states
  • Fixes for Q-Chem EOM-CC and fchk
  • Fixes for Columbus, Turbomole
TheoDORE – Download

Download the newest release of the TheoDORE wavefunction analysis program – TheoDORE 3.1.1 (23 June 2023)

Size: 12 MB
Version: 3.1.1

Full release notes

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Excited-state aromaticity in naphthalene

A recent JPCA article by Karadakov and Al-Yassiri highlights the differences in singlet and triplet aromaticity in naphthalene. To me this paper contains several striking observations:

  • The singlet HOMO/LUMO transition (S2, 1La) is shown to be strongly aromatic whereas the triplet HOMO/LUMO transition (T1, 3La) is antiaromatic. Does this mean states reached by the same kind of orbital transition behave differently depending on their spin-multiplicity?
  • The aromatic S2 lies above the antiaromatic S1 even though S2 is the HOMO/LUMO transition. Does this mean that singlet antiaromaticity is actually a stabilising effect?

We have discussed the excited states of naphthalene from an entirely different viewpoint in a recent J. Chem. Theory Comput. article. It would be fascinating to combine the two viewpoints.

HOMO/LUMO transitions

We just posted a preprint discussing a question I have been wondering about for a while: Why is the lowest excited state of a molecule not always the HOMO/LUMO transition? More generally we show how singlet and triplet state energies are affected in different ways by post-MO energy terms.

The preprint can be found here: Excited-state energy component analysis for molecules – Why the lowest excited state is not always the HOMO-LUMO transition

Update: the final published version is available here.