Ab Initio Multicomponent Methods
An accurate inclusion of nuclear quantum effects is important for a correct description of many chemical phenomena including zero point energy, hydrogen tunneling, and proton-coupled electron transfer. One promising class of methods for including nuclear quantum effects in quantum chemistry calculations are multicomponent methods, in which electrons and select nuclei are both treated fully quantum mechanically by not invoking the Born-Oppenheimer approximation for the select nuclei.
We have developed a wide variety of multicomponent methods including the first multicomponent generalization of the (T) correction and the first analytic nuclear energy gradient in multicomponent coupled-cluster theory. Our current research interests include developing new multicomponent methods in combination with the constrained NEO formalism and the application of multicomponent methods to hydrogen-bonded and proton-transfer systems.
Ab Initio Polaritonic Methods
Placing molecules inside optical cavities can create polaritons, which are hybrid states of light and matter. Polaritons have been reported to change chemical reactivity, energy transfer, and spectroscopic properties. To describe these systems from first principles, the electrons and the photons of the cavity mode must both be treated quantum mechanically. Quantum electrodynamics coupled-cluster (QED-CC) theory does this. It extends coupled-cluster theory, one of the most accurate methods in quantum chemistry, to include photonic excitations and electron–photon correlation. This makes it a natural tool for predicting how a cavity changes the properties of a molecule.
Our group develops QED-CC methods that connect ab initio polariton theory to the quantities measured in quantum optics and polariton experiments. We performed the first calculations of intracavity photon statistics from QED-CC theory. We derived an exact bound on how molecules squeeze the vacuum field of a cavity. We also added cavity loss to real-time QED-CC theory to obtain polariton linewidths and driven steady states. Our current research interests include dissipative (open-system) QED-CC methods, cavity-induced changes in molecular response properties such as polarizabilities, and extending QED-CC to the collective coupling of many molecules found in experiments.
Ice Lithography
Ice lithography is a form of electron-beam lithography in which a thin film of condensed molecular ice, such as water or a simple alcohol, serves as the resist. The film is deposited and patterned at cryogenic temperatures and developed by warming rather than with solvents. This makes the technique attractive for patterning fragile or non-planar substrates. In organic ices the electron beam drives bond cleavage, radical recombination, and cross-linking. The competition among these reactions determines whether exposed regions are removed or left behind as a non-volatile residue. These mechanisms are difficult to resolve experimentally, and choosing resists and process conditions rationally requires a molecular-level model.
In collaboration with the experimental group of Gavin King, we have developed the first species-resolved kinetic model of ice lithography with alcohol resists. The model uses rate equations for the radiation chemistry of the ice and treats cross-linking as a gelation process. It reproduces the dose-dependent tone of methanol and the carbonization of ethanol seen in experiment. We have also applied it to film development, to the effects of dose rate and substrate temperature, and to screening new resist molecules. Our current research interests include computing reaction barriers and branching ratios for these ices with electronic structure methods, so that the model’s rate constants come fro first principles, and using the model to design resists and process conditions.