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.
There are about four or five distinct ingredients used in modern density functionals:
- integrals of functions of the density and its gradients (GGAs, etc)
- inclusion of the kinetic energy density in such expressions (meta-GGAs)
- a fraction of exact Hartree–Fock exchange (or a length-scale-screened variant)
- a contribution from a wavefunction-based correlation method like MP2 or RPA
Now there is another:
Tim Wiles and F R Manby, ‘Wavefunction-like Correlation Model for Use in Hybrid Density Functionals’, J. Chem. Theory Comput., Article ASAP, DOI: 10.1021/acs.jctc.8b00337.
In this paper we introduce the UW12 correlation functional:
- an explicit functional of occupied orbitals (i.e. a functional of the 1-RDM)
- an F12-like model of dynamic electron correlation
- can be self-consistently optimized in a straightforward way
- most of the complicated stuff handled through familiar DFT quadrature
- no double-summation over virtual orbitals, so fast basis-set convergence
This functional can be used as a viable (and hybrid) alternative to use of MP2 in double-hybrid functionals, as we demonstrate in this paper. Our hybrid XCH-BLYP-UW12 functional has only one parameter, and is competitive with more highly parameterised double hybrids for reaction barrier test sets.
If you don’t have access and want to read the paper, just email Fred.
Congratulations to Tim for publishing this work!
Alex, Fred, Tim and Peter
A small sub-group had a great hike in the Brecon Beacons in Wales on Monday!
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!
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.
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!
Felix’s work on Markovianity in photosynthetic energy transport has been published: Felix Vaughan, N Linden and F R Manby, J. Chem. Phys. 146, 124113 (2017); http://dx.doi.org/10.1063/1.4978568.
Modelling the interaction between excitons and the surrounding environment is a non-trivial problem. Many interesting insights about excitonic energy transfer have made use of a Markovian or “memoryless” approximation to this interaction. In this paper we assess the applicability of this approximation by employing a new metric of non-Markovianity.
We find that for smooth spectral densities the Markovian approximation works well provided that a precise change to the system Hamiltonian is made, which for the dimer system studied corresponds to an increase in the coupling strength between chromophores. We also find that discrete vibrational modes resonant with the eigenstates of the Hamiltonian induce the greatest degree of non-markovianity. Ultimately we conclude that to model exciton dynamics coupled to realistic spectral densities a Markovian approximation is not suitable.
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.
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.
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.
Through 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.
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
University of Bristol EPSRC Doctoral Prize Fellowships
There is a great opportunity to join the Manby group if you have recently finished (or are just about to finish) an EPSRC-funded PhD project in the UK. The University of Bristol is advertising prestigious one- to two-year Doctoral Prize Fellowships for outstanding applicants.
The application deadline is 31st October, and if this opportunity interests you, please email Fred to discuss potential projects.
TMCS Centre for Doctoral Training
We are now seeking applications to join our fourth cohort of students in the Theory and Modelling in Chemical Sciences CDT. Our students study theory, modelling and software development together in year one, based in Oxford. Years two-four are devoted to the main PhD project in one of the many research groups associated with the Centre.
Interactions between Drude oscillators (two opposite charges, one fixed, one mobile, interacting by a spring) are almost always modelled in the dipole approximation. This is because the Hamiltonian for describing Drude oscillators in this way is quadratic in the creation/annihilation operators (or equivalently, is quadratic in position and momentum) and can therefore be diagonalized exactly through a normal-mode transformation.
Second-order perturbation theory for the interaction of two such oscillators produces the expected –C6 R–6 dispersion interaction. At short distances this diverges, and the exact interaction energy ceases to be defined at all.
Therefore people use screening functions to fix up the short range. How much are the screening functions fixing up the divergence, and how much are they just compensating for the use of the dipole interaction Hamiltonian? Now we can find out, because Mainak has calculated the (almost) exact binding energy between Drude oscillators with the full Coulomb interaction.
Quantum mechanics of Drude oscillators with full Coulomb interaction
M. Sadhukhan and F. R. Manby, Phys. Rev. B 94, 115106 (2016) 10.1103/PhysRevB.94.115106