Department of Chemistry

1) Coupled Cluster Methods and Polarizable Solvation Models

Example #1 (Larger view)
Example #2 (Larger view)

Most chemistry happens in solution, and the solvent has a profound influence on reactivity and molecular properties. The solvent has a direct effect on the geometry of the solute, for instance stabilizing structures that are not stable in gas phase as it is the case for glycine, which is a zwitterion in water because water stabilizes charge separation. The solvent also have a direct effect on properties through the polarization of the wave function, for instance different solvent generate different solvatochromic shifts of absorption and emission spectra.
However, most quantum mechanical (QM) calculations are performed in gas phase since accounting for the large number of solvent molecules at QM level is basically impossible. Additionally, accurate QM methods like those belonging to coupled cluster (CC) theory scale very unfavorably with system size so that any extra molecule increases the computational cost considerably. Solvation effects can be introduced efficiently by considering classical models, either implicit or explicit, that can polarize and be polarized by the wave function of the solute. We develop strategies to couple CC methods with these solvation models, aiming at the best compromise between accuracy and computational effort. We consider both implicit and explicit polarizable solvation models, shown in Figure, for ground and excited state molecular properties. Our goal is to take advantage of the high-quality of the CC results with the computational efficiency of these solvation models to simulate the spectra of complex chromophores in bulk solution or at the interface between solid and liquid phase.

2) Mutually Polarizable QM/QM Embedding Schemes

Example (Larger view)

When molecules are too large to be treated entirely at an accurate QM level, hybrid methods are often employed to reduce the computational cost. In hybrid methods, the system is separated in layers where the most important region is treated with a high level of theory while the rest is treated at a lower, more manageable level of theory. The success of this approach is often connected with the ability to treat the interaction between the layers appropriately. This becomes even more important when we want to study electronic excited states because of the more polarizable nature of the electron density.
We are working on developing methods to account for mutual polarization between high and low levels of theory in a self-consistent manner, see scheme in Figure. Starting from the ONIOM (Our own N-layer molecular Orbital molecular Mechanics) method of Morokuma for excitation energies, we are working on defining polarizable embedding fields whose parameters are computed on-the-fly from the low-level calculation. Additionally, we are developing techniques to extrapolate multiple excited states simultaneously to avoid the problem of state-matching between different levels of theory.

3) Orbital Analysis of Molecular Optical Activity Based on Configuration Rotatory Strength 

Example (Larger view)

The ability of optically active media (molecules or solids) to rotate the plane of polarization of linearly polarized light has been known for over two hundred years, since the work of Biot and Arago at the beginning of the 19th century. Since then, experimental and theoretical techniques have made enormous strides in the ability of measuring and computing optical activity in all of its various forms (refringence, absorption, and scattering). And yet, we still have no idea about the structure-property relation for optically active compounds. In other words, nobody is able to predict the sign and magnitude of the optical rotation induced by an optically active compound just by looking at its structure.
We are developing methods to try to understand this elusive structure-property relation by separating the optical rotation tensor in excited configuration contributions. These contributions are a subproduct of standard linear response calculations, and can be combined to define a rotatory strength in configuration space (the rotatory strength is the equivalent in circular dichroism spectra of the oscillator strength in absorption spectra). An example of the contributions for (P)-2,3-pentadiene is shown in Figure. These results show that only a limited number of significant contributions are necessary to compute the optical rotation. Therefore, this approach may help us understand which electronic interactions carry the largest weight for the final value of the optical rotation, and may provide the key to this longstanding puzzle.

4) Phenyl-Thiopenes Excited State Electronic Structure

Example (Larger view)

This project is in collaboration with the Elles group. We are studying the electronic structure of an interesting class of prototype molecules: phenyl-thiopenes (PTs). PTs are model systems for more complicated thiophene polymers that have many interesting applications in materials science. We are interested in studying how absorption of light affects the excited state dynamics both from the experimental and theoretical points of view. We are therefore evaluating ground and excited state singlet and triplet potential energy surfaces, and simulating IR and Raman spectra on all surfaces, see Figure for diphenyl-thiophene. The information gained on these molecules will be then extended to more complicated compounds in solution and in solid phase.

5) Electron Transfer Coupling on Surfaces

Example (Larger view)

Electron transfer (ET) from an excited chromophore to organic crystals is an important process for organic photovoltaic devices. However, the evaluation of the transfer probabilities is hard due to the extended nature of the systems involved. In this project in collaboration with Chan group, we are investigating a Zn-phthalocyanine (ZnPc) film deposited on a graphene sheet. We are evaluating the ET coupling integrals from the ZnPc to single- and double-layer graphene using both a cluster approach and periodic boundary conditions (PBCs), shown in Figure. We are currently developing a method for the efficient evaluation of such coupling integrals for PBCs that has general applicability.

 

 


News

12/15/20 Robert and Ty join our group, welcome!

12/14/16 Sijin and Joseph's (Lawrence High School) paper on EOM-CCSD-PCM benchmarcking was accepted in JCTC

11/27/16 Alessandro's paper with Jeremy, our REU student from summer 2015, on point charge embedding for excited states was accepted for publication on JCP.

10/26/16 The entire Caricato group (except Marco) is presenting at MWRM2016 in Manhattan, KS.

10/20/16 Tal's paper on helicene derivatives in collaboration with the Avarvari group in France is accepted for publication in Chemistry-A European Journal!

08/01/16 Alessandro's paper on electronic coupling in solids using DFT PBC methods is published on JPCC.

05/09/16 Sijin's paper on multi-state extrapolation of UV/Vis spectra is published on JCP.

05/07/16 Matt and Sijin receive the McCollum Award for excellence in research. Congratulations!

12/05/15 Matt and Alessandro give a talk at the Kansas Physical Chemistry Symposium, while Tal and Sijin present a poster. 

12/03/15 Amy Jystad joins our group as a graduate student. Welcome Amy!

11/06/15 Marco's paper was accepted in JCTC

 

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