This group is dedicated to a fundamental understanding of the nature of electron correlation.

Projects include developing efficient, high-accuracy wavefunction-based methods for large systems, studying the x-ray spectroscopy of solids (static) and isolated molecules (time-resolved), and investigating the fundamentals of correlated electronic dynamics. All of these projects should culminate in better understanding of chemical reactions, including energy and electron transfer, particularly in the condensed phase and at interfaces.

One of the major shortcomings of modern fragment-based electronic structure methods is that electron correlation (if included) is handled at the level of individual electrons. This project consists of two theoretical pieces. In the first, the Hamiltonian for a supersystem is rigorously renormalized to be in terms of fluctuations of entire subsystems (among internally correlated electronic states). In the second part, familiar electronic structure methods, notably coupled-cluster theory, can be applied to this renormalized Hamiltonian. Initial tests of accuracy per cost are quite promising. The next steps are to begin applying this model to systems of wider interest and implement the theoretically straightforward generalization to excited states.

See publications 26 and 27.

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One elementary open problem in the electronic structure theory of solids is the source of extra background electrons in x-ray photoelectron spectroscopy. Removal of this background component is phenomenologically understood, and it must be done for compositional analysis. However, its contribution to the total electron flux is not accounted for by direct cross-sections, and its shape is not accounted for by energy losses as electrons traverse the solid. In collaboration with an experimental group, we proposed an interesting many-body effect by which some of the total flux can be accounted for by a kind of intensity borrowing from a deeper core polarization; energy losses to the valence from this transient local oscillator give an unusual background shape. This hypothesis is supported well by the experiments of our collaborators. We are now working on an approach to semi-quantitative (trend-predictive) simulation.

See publication 25.

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As photon energy increases, so does the available bandwidth. This makes possible extremely short pulses, from just a few femtoseconds down to tens of attoseconds. This is sufficient resolution to probe the fastest vibrations and even typical electronic motions in real time. This technology is presently being developed only in the most advanced laboratories around the world, but as expertise grows and spreads, it will reface our understanding of chemical and electronic dynamics. Such studies are in desperate need of new theoretical tools, since existing quantum chemistry packages focus mostly on the ground state and a handful of low-lying excited states, well below the dense manifold of states at x-ray energies. We are currently developing the theoretical framework and associated computational tools to support these advanced experimental explorations.

See publications 20, 22, and 24.

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Real-time quantum dynamics studies are a mainstay for nuclear motions of complex systems. The reason for this is that the density of states at relevant energies renders the eigenstate formulation computationally intractable. We should not expect the situation to be any different as we consider ever more complex electronic systems, for example charge and energy transfer in photosynthesis. However, most electronic structure packages are only equipped to work within the eigenstate formulation. There are many unanswered questions about how electron correlation affects the salient features of electronic dynamics. Furthermore, relative to vibrational dynamics, analysis is complicated by the indistinguishability of those electrons that move from those electrons that do not. Our work in this area addresses the dual challenges of simulating correlated many-electron dynamics and interpreting the results.

See publications 20, 21, and 22.

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