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
Jenna and Felix doing sums
Ed and Takashi writing code
Bristol Chemistry undergrads Jenna Ram and Ed Smith have joined the group for summer projects. Jenna is working on mean-field and perturbative approaches for model Hamiltonians describing systems that contain both fermions and bosons. Ed is working on optimization of density-functional quadrature grids in our in new code.
Our paper A Projector-Embedding Approach for Multiscale Coupled-Cluster Calculations Applied to Citrate Synthase has appeared online in JCTC, and was one of the most highly downloaded papers soon after publication.
Citrate synthase active site and QM/MM reaction profiles computed with DFT and with CCSD(T)-in-DFT for various approximate exchange-correlation functionals.
The great thing about this is that by tuning in CCSD(T) for just a handful of reacting atoms, almost all of the dependence on the approximate exchange-correlation functional is eliminated.
We’re excited to be welcoming around half of our current TMCS cohort of students to the CCC in Bristol on Monday, and the other half the following week.
The first batch of students will be with us for the week, talking science with academics and group members, and trying to decide what they’re going to work on for their PhD project.
TMCS 2015 cohort
The second group of students will be working on 5-week short projects with all of the groups in the CCC.