Category Archives: Group news

Welcome to new group members

We’re delighted to welcome Susannah Bourne-Worster and Zack Williams to the group!

Susannah just completed her DPhil with Prof Peter Hore at the University of Oxford, and was awarded a prestigious three-year fellowship by the Royal Commission for the Exhibition of 1851. She will be working on our project on understanding efficiency of energy transport in photosynthesis.

Zack completed year-one of the TMCS programme and has joined the group for his doctoral research. He will be picking up Tim’s project on correlation functionals using the Unsöld approximation.

Welcome new group members!

Five people have (fairly) recently joined the group: Callum Bungey and Rocco Meli as graduate students through the TMCS programme; Alex Buccheri as a PDRA working with my colleague Neil Allan and me on the CCP5 flagship project;  Thomas Dresselhaus as a PDRA working on the entos project; and Fidel Batista Romero as a Royal Society Newton International Fellow. Apologies for not recording these arrivals earlier!

 

Welcome to summer students

This year we have four summer students in the group, working on projects ranging from basic theory of quantum polarization models through to biological applications of projector-based embedding methods.

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Left to right: Fred, Shubham, Dom, Aidan and Rebecca. (And yes, Dom’s eyes were closed in all of the shots.)

Shubham joins us from IIT Kharagpur, India, where he is studying Chemistry. Dom and Aidan are Bristol chemistry undergraduates, and are both funded by RSC Undergraduate Bursaries. Rebecca is studying on a combined Chemistry with Maths degree at the University of Southampton.

Welcome to the group!

Delocalization errors in WF-in-DFT embedding are large, but fixable

Delocalization error in approximate DFT clearly manifests itself in homodimer cation systems (like H2+ or (H2O)2+), with GGA functionals typically leading to large energy errors and qualitatively incorrect structures. It also causes problems in a variety of other chemically important contexts.

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Spurious delocalization of spin density in a small radical-cation water cluster.

We have found that the delocalization error in densities can cause major errors in WF-in-DFT embedding – these errors are not particular to the projector-based scheme we use,but simple expose a limitation of partitioning systems based on the electron density when that electron density is qualitatively flawed.

Following work from Kieron Burke, we have found the simple expedient of using Hartree-Fock densities in WF-in-DFT calculations really improves reliability in cases where there is a serious delocalization error, and doesn’t cause major problems (in the examples we have studied) when there is not a big delocalization error.

You can read about this work in a paper that has just appeared online: Pennifold et al., ‘Correcting density-driven errors in projection-based embedding’, J. Chem. Phys. 146, 084113 (2017); DOI: 10.1063/1.4974929.

Pump-probe experiments on photosynthetic light-harvesting complexes are not easy to interpret

Clem’s paper on the interpretation of pump-probe experiments on the purple-bacteria light-harvesting complex LHII is now in print in J Phys Chem B.

jp-2016-09916z_0008Through careful and extensive calculations involving molecular dynamics, time-dependent density functional theory, and quantum dynamics we have shown that the interpretation of anisotropy decay rates in terms of strength of coupling to a dissipative bath is not so easily justified. The reason is that static (or inhomogeneous) disorder itself produces anisotropy decay at about the experimentally observed rate.

The paper also contains an epic, paper-length appendix on how to compute such quantities for the circularly degenerate oscillator model.

Congratulations Clem!

C. Stross, M. W. Van der Kamp, T. A. A. Oliver, J. N. Harvey, N. Linden and F. R. Manby, “How Static Disorder Mimics Decoherence in Anisotropy Pump–Probe Experiments on Purple-Bacteria Light Harvesting Complexes”, J. Phys. Chem. B, 120, 11449-11463 (2016), DOI: 10.1021/acs.jpcb.6b09916