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.
The latest release of Molpro provides a range of new functionality, including methods developed in the Manby group
A key new addition is wavefunction-in-DFT embedding through the projector method with basis-set truncation. This enables straightforward embedding of practically any electronic structure method implemented in Molpro in a chemical environment modelled by DFT. Unlike many embedding methods of this kind
there are no issues with partitioning across covalent bonds.
We have performed many calculations using the combination CCSD(T)-in-DFT, with the expensive coupled-cluster calculation only on a few, chemically active atoms. The exciting opportunity here is that the method all but eliminates dependence on choice of exchange-correlation functional.
The research behind this functionality was carried out by Martina Stella, Simon Bennie and Rob Pennifold in the Manby group; and by Jason Goodpaster and Taylor Barnes in Tom Miller’s group at Caltech.
Distinguishable cluster theory
Distinguishable cluster theory with singles and doubles (DCSD) is a simple modification of CCSD, but with remarkable properties, including the ability to dissociate molecules correctly even with a closed-shell reference state.
The method was developed by Daniel Kats while he was a DFG-funded postdoc in the Manby group. There is much discussion about what exactly this approximation means (but see Daniel’s interesting paper about a screened Coulomb derivation). Whatever it turns out to mean, DCSD is already a useful tool and a powerful addition to the set of available methods. Now in Molpro running it is as simple as replacing ccsd with dcsd in the input (and the computational cost is the same as CCSD). Gradients as well as F12, orbital-optimized and Brueckner variants are available.
Thanks to the efforts of Dr James Womack, we now have Intception, an incredibly powerful framework for automatic implementation of Gaussian integrals from equations that define the recurrence relations.
More on this later – but feel free to look at the information online and use Intception to generate integrals for your project.
Intception is open source, but as with a compiler, you can generate inputs and outputs under any licensing model you like.